We use "who" when referring to the subject of a sentence or clause, here the word "people" is the subject of the clause "the most valuable resources provided by coworking spaces are actually the people whom use
them." The correction of the underlined part will be "who use".
'Whom' is an object pronoun that does not perform the action 'use'.
Hence, Option D is correct. and A is incorrect.
Options B and C: 'Uses' is a singular verb that follows a singular subject, whereas the subject here 'people' is plural.
Hence these options are incorrect.