If you want to be strict then Lady C's maiden name is 'A' going by the definition of maiden.
However, in this specific case the lady has changed her surname from A to B through choice, not marriage, possibly using a system similar to the UK Deed Poll. In the UK a clause given by the UK Deed Poll Service would apply which explicitly disallows the use of a previous registered name.
From UK Deed Poll Service
Two of the declarations you make on your Deed Poll are that you will entirely abandon the use of your old name and you will only use your new name in future. [http://www.deedpoll.org.uk/OtherFAQs.html]
In that case Lady A would be forced to use Lady B as her maiden name, however it is rare that birth certificates are changed after a Deed Poll name registration so her birth surname would remain as A. (This restriction may vary from country to country depending on local laws and regulations.)
In a more general case...
Of course, any lady can choose what she wishes to write as her maiden name but the meanings and uses of the word maiden, suggest otherwise.
Unless otherwise noted all quotes are from OED1.
Maiden : 1. A girl; a young (unmarried) woman. 2. A virgin 3. An unmarried woman, spinster [and some others not related directly to women or marriage]
adj. (from appositive and attributive uses of the n.). Cf. virgin. I. Literal uses.
1. Appositive uses. a. Unmarried; now chiefly in maiden lady, maiden sister: see also maiden aunt. †b. Of a child: Female; see maiden-child (obs.). c. Virgin; sometimes said of men (obs.). 2. Of or pertaining to a maiden, or to maidenhood; befitting a maiden, having the qualities of a maiden. maiden name: the surname borne by a married woman before her marriage.
Unmarried here is the issue does it mean was married and now no longer is? ... the definition isn't entirely clear.
Unmarried : 1. Of persons: Not married, unwedded. 2. Lived free from marriage.
Maiden defn2. A virgin has spawned a couple of maiden ..... words
Maidenhead : 1. The state or condition of a maiden; virginity; said occas. of a man. [and one more about the first crop of fruit]
Maidenhood : The condition of being a maiden; the time of life during which one is a maiden. in phrases to have, hold, keep, lose, etc. (one's) maidenhood.
Fairly clear that maiden here means a virgin, but that doesn't stop Miss A marrying Mr B and still remaining a virgin (much to Mr B's dismay one would imagine and possibly the reason why Mr B might file for divorce making Mrs B unmarried again and free to tie the knot with the unwitting Mr C) but Miss A is now called Miss B so when she marries Mr C is her maiden name A or B?
More maidenish things could sway me towards her maiden name being A. These are some of the other definitions for Maiden that I skipped earlier on.
Maiden : (figurative) 4. That has yielded no results. 5. That has not been conquered, tried, worked etc. [the etc. runs to "A castle that has never been taken;'virgin'. [stuff about fruit again]. Of soil or metals, that has never been disturbed ploughed, or worked. Of a soldier or his weapon: untried. 6. That is the first of its kind; made, used, etc. for the first time. Occas. in sense early, earliest.
Excluding the recurring fruit mentions, defn. 5 seems pretty much the same as virgin to me - not been conquered, tried, worked, disturbed, ploughed all could easily sound a little like euphemistic terms.
But it is defn.6. that seals it. The first of it's kind and there are a number of Maiden .... firsts.
Quotes from Wikipedia
Maiden flight - the first occasion on which an aircraft leaves the ground under its own power.
Maiden voyage - the first journey made by the craft after shakedown. A number of traditions and superstitions are associated with it.
Maiden speech - the first speech given by a newly elected or appointed member of a legislature or parliament.
I think a combination of unmarried, virgin, and first pretty much marks maiden name down as the first surname a lady had.
Interestingly, defn.4 : That has yielded no results gives us
A maiden over : Cricket, of an over: One in which no runs are scored.
A maiden horse : one which has won no prizes.
A maiden tide : one where no vessels enter or exit a harbour
A maiden circuit : Of an assize, circuit, session: Formerly, one at which no prisoner was condemned to death; now, one at which there are no cases for trial.
Wikipedia contributors, "Maiden speech," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Maiden_speech&oldid=626049739 (accessed September 27, 2014)
Wikipedia contributors, "Maiden voyage," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Maiden_voyage&oldid=620824032 (accessed September 27, 2014).
Wikipedia contributors, "Maiden flight," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Maiden_flight&oldid=619347591 (accessed September 27, 2014).