The correct option is
B Whom
In order to combine the two sentences, we need a relative pronoun. Let's first understand what relative pronouns are. Relative pronouns are pronouns, used to link/combine clauses or phrases in a sentence. For example, 'I don't know who you are.' Here, 'who' is used to connect the phrase 'I don't know' with the rest of the sentence. Also, 'who' connects the clause with 'you' here.NOTE: Here, 'us' is the objective pronoun and hence we need an objective relative pronoun to relate the two clauses/sentences.
Option B - 'Who' is a relative pronoun, which is used to relate people/nouns with the other clause/phrase. It is often used with the subjective pronoun/noun. As the noun/pronoun which has to be related is objective, we can't use 'who' here. Hence, option B is incorrect.
Option C - 'That' is a relative pronoun which is used to demonstrate people or things that aren't practically near/close to hand. As the sentence isn't about any demonstration, we can't use 'that' here. Hence, option C is incorrect.
Option D - 'Which' is a relative pronoun, which is used for the things/ideas that have been previously mentioned. Hence, option D is incorrect.
Option A - 'Whom' is a relative pronoun which is used to relate an objective noun/pronoun with a clause/phrase. As the pronoun 'us' is objective, we can use 'whom' here. Hence, option A is the correct answer.
'Our relatives, whom we trusted totally, have not failed us.'