The answer to your question would be that the homophone pair that needs to be corrected is the following one: To/too. That is, the correct option would be D.
The first "to" of the sentence should be the preposition "to" because we can see that it is to be followed by the NP "the concert". "Too" is an adverb which, as such, does not take an NP as object. Therefore, the revised version of the sentence would be as follows: If you're going to the concert, may I ride in your car too?