Although A. is considered acceptable in informal English, it is widely considered incorrect, since it says "better ... than me" when it should say "better than I"
B. is incorrect. "who" refers to the subject of the sentence, while "whom" refers to the object of the verb. To test which one to use, trying putting him/her or he/she in the sentence. if he/she works, then "who" should be used. If him/hers works, then "whom" is correct. In B, him/her would work:
(i turned the question into a statement for this 'test' to work)
"You sold the old car to him/her."
To sum all of this up, the question should be "To whom did you sell your old car?"
C. is correct; woop woop! "us" is used correctly in the sentence (as the object of the preposition).
Unlike C, sentence D. uses "us" incorrectly. It is used to refer to the subject (students) instead of being used as an object of the verb. To make this sentence correct, you would just replace "us" with "we"
--I hope you found this helpful! If you did, mark this answer as brainliest! (it would make my day :) --
