A defining relative clause gives essential information about someone or
something, whereas a non-defining relative clause gives extra or
unrelated information about someone/something.
The clause "which I bought from Delhi" is a relative clause that defines the noun
"pants" by pointing out which pants. Hence it is the defining clause and option A is correct.
Option B is incorrect because this forms the predicate of the sentence (that begins with the main verb "don't fit) whereas the relative clause does not contain the main verb.
Option C is incorrect because it isn't a relative clause, it contains the subject "The pants" and a part of the relative clause "which I bought".
Option D is incorrect because this is a main clause, excluding the relative clause.