[This is an important question because of all the folks visiting this site for guidance, who may well pass answers on to other students and writers. I myself in my pre-linguist days used to fall victim to this 'rule'. I have total sympathy for the Original Poster, as I do for my former self, and all literature, EFL students and authors who are confronted with this so-called grammar rule. GMAT students don't despair, just find which non-existent rule they're looking for now. It's quite good fun and not that difficult. Anyway here's my answer to the question...]
The idea that which is not used for restrictive clauses is a myth promulgated in the worst English grammar text-books and style guides ever written. The greatest writers in the English language have continuously used which as a relative pronoun in restrictive relative clauses — as has everyone else too.
Some info from the post Sidney Goldberg on NYT grammar: zero for three, by Geoffrey Pullum, Professor of General Linguistics at the University of Edinburgh, co-author of the Cambridge Grammar of the English Language (2002), shows how far through various books you would need to go before finding which appearing as a restrictive relative pronoun. The first number given in the list below shows the number of lines in the entire book. The second number shows on which line the author first used which as a relative pronoun in a restrictive relative clause:
- A Christmas Carol (Dickens): 1,921 lines, first occurrence on line 217 = 11% of the way through;
- Alice in Wonderland (Carroll): 1,618 lines, line 143 = 8%;
- Dracula (Stoker): 9,824 lines, line 8 = less than 1%;
- Lord Jim (Conrad): 8,045 lines, line 15 = 1%;
- Moby Dick (Melville): 10,263 lines, line 103 = 1%;
- Wuthering Heights (Bronte): 7,599 lines, line 56 = 0.736%...
Now, I know these authors are idiots who didn't know how to speak English properly, but just how did their editors and publishers get away with it? And all the people who decided to put these books on school syllabi? They, of course, should be shot.
Getting serious again: importantly, as Pullum also shows in the post A Rule Which Will Live in Infamy, there are situations when we, in fact, cannot use that for restrictive clauses and in which we have to use which. (Oh look one of them happened right there. I couldn't for example have written ... "and in that we have to use which"). Here are the three situations Geoff Pullum describes:
- The putative ban can’t apply when a preposition precedes the relative pronoun: The town in which she lived is grammatical but *the town in that she lived isn’t.
- The supposed rule should be ignored when modifying demonstrative that, because that which you prefer is clearly preferable to ?that that you prefer.
- The rule can’t apply to a conjoined which: We must trust the unknown entity who or which created us is grammatical but *We must trust the unknown entity who or that created us isn’t.
Here is the passage in which (Oh no, there I go again with another one!) the famous quote that Pullum is playing with appears:
“Yesterday, December 7, 1941—a date which will live in infamy—the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan.” That was how President Franklin D. Roosevelt opened his famous infamy speech, 71 years ago. Ignoring the writing handbooks, he opened with a passive construction, which of course is just right for the rhetorical context (America as innocent victim). And he also ignored another bogeyman rule: He introduced a restrictive relative clause with which.
The answer to the OP's question, therefore, is that which can, and sometimes must, be used for restrictive relative clauses. It's not a mistake to do so.
I leave you with Geoff Pullum's last words from the link above:
Grammar snobs trying to show off their linguistic rectitude by playing gotcha with an invented rule that never matched educated usage; copy editors slaving away trying to enforce it; Microsoft Word blindly putting wavy green underlining under every relative which not preceded by a comma. What a senseless waste of time and energy.
Follow the Fowler rule if you want to; it’s up to you. But don’t tell me that it’s crucial or that the best writers respect it. It’s a time-wasting early-20th-century fetish, a bogeyman rule undeserving of the attention of intelligent grownups.