The correct option is D man
An adjective clause is a group of words that contain a subject and a
verb, and it functions as an adjective that modifies a noun or a
pronoun. Adjective clauses usually begin with relative pronouns such as
who, whom, whose or a relative adverb like when, where and so on.
The adjective clause is generally placed right after the noun or pronoun it modifies.
Option C is correct because the adjective clause "whom we met in the subway" describes the "man".
Option A is incorrect because the clause does not tell us 'which sister', it tells us 'which man'.
Similarly, option B is incorrect as the clause does not tells us more about 'fiance', even though the fiance is the man, but it specifically defines or describes the noun 'man'.
Option D is incorrect because 'my' is not a noun, it is a determiner.