Both right, with a little nuance as will be explained later.
When had he proposed to her? In the 1970s or 1980s?
is using the "pluperfect" or otherwise named "past perfect tense", ie., "he had proposed". Because this is interrogatory there's an inversion of "he" and "had", which is common.
formed in English by had and the past participle, as in
he had gone by then
Oxford Living Dictionaries
Your other sentence:
When did he propose to her? In the 1970s or 1980s?
This is in the simple past tense: "he proposed to her". Though instead of a simple inversion in the question form, we use the auxiliary verb "did", and the verb changes to the infinitive form, ie., "did he propose?" rather than "did he proposed?".
As to whether pluperfect and simple past have different meanings, they do, but it's not always a distinctive difference. In this context I can't see much of a distinction. I think this is because the distinction of the pluperfect and past simple is one of denoting a continuing action versus a single act done once, and in this context "proposing" is taken as being a single action done once and not continuously.
However take the previous example sentence:
"He had gone overseas by then."
(uses the pluperfect, correct)
"He went overseas by then."
(uses the simple past tense, nearly
This is because the act of "going overseas" in this context is taken as a single act, not a continuing one.
Something like "he went overseas" would imply something like he regularly went overseas.
So in other words there is a difference in the two tenses, but in some cases there's no or little distinction to be made, as in your case I think.