I couldn’t open your link. What was the correct answer choice for the OP’s original question?
I agree the answer choices the OP gave were both wrong because they had “strongest” instead of “stronger”.
Can you give any official SAT questions in which “which” is used to refer to a whole clause in the correct answer choice?
I have never seen one.
I have seen plenty of questions in which “which” is used to refer to a clause in an incorrect answer choice.
For example: OG Test 3 Section 10 Question 7 choice E
"Their flight was missed, which eventually caused the bride’s parents to run etc. " (Answer choice incorrect)
OG Test 7 Section 10 Question 4 choice B
The information age has ushered children into a global society, which is the cause of educators lamenting a lack of texts that explain the diversity of cultures. (Answer choice incorrect)
also
OG Test 8 Section 4 Question 9 Choice B (incorrect)
OG Test 8 Section 10 Question 12 Choice C (incorrect)
Probably you were able to avoid making errors on your tests because “which” referring to a clause generally occurs on the SAT together with other errors, and you must have picked up on these other errors. But I would be really, really surprised if you could come up with that usage in a correct answer choice. This is not SAT usage.
“Which” to refer to a clause is not really like a contraction. It is more like the use of “that” as a relative pronoun for people: an ungrammatical usage that is so common that some authorities think it is acceptable.