Final answer:
The correct answer is 'Whom' as it is used as an object pronoun in the given sentence, which is structured in a passive voice.
Step-by-step explanation:
The correct answer to the question "By who/whom were telephones improved?" is option B, "Whom". In English grammar, whom is used as an object pronoun, which is the correct form to use when the pronoun is the object of a verb or preposition. In this case, whom refers to the individuals that improved the telephone, and the sentence implies a passive construction where whom is not the subject performing the action of improving, but rather the object that is receiving the action.