Relative pronouns are the pronouns that are used to introduce the
dependent clause in the sentence. It can be for either the object or the
subject.
Here, in this question, the relative
pronoun with which we are supposed to fill the blank should correctly
represent the subject ("food").
Thus the most appropriate word from among the options is "which".
The relative pronouns used for objects are: "which", "that", "whose".
"whom" & "who" are used to refer to people, thus it is inappropriate for use here, and this eliminates option B & C.
"when" is used to refer to time, thus it is inappropriate for use here, and this eliminates option D.
Therefore the correct complete sentence is:
I don't like food WHICH is very spicy.