The correct answer is Option C.
We are to choose the correct interrogative word from the given options.
We can make out from the sentence that the speaker has to choose a dress from a definite set of various dresses.
So the correct answer is the word that is used when one is to ask for information or things from a definite set.
Option A- HOW is used when one asks about a method or manner of something.
Option B- WHO is used when one asks about a particular person.
Option D- WHEN is used when one asks about the time of the occurrence of an event.
Hence these options are not the desirable answers since they are not used to ask for a particular thing from a set.
Option C-WHICH is used when one has to choose from a definite set. Here the speaker will use the word 'which' because he or she wishes to choose a single dress from various dresses.
Therefore Option C is the correct answer.