Answer:
B). That assignment, which was due today, was very difficult.
Step-by-step explanation:
A relative pronoun is employed to link a relative clause or phrase to a noun. This clause describes/modifies a noun that is located right after the noun. For example: who, which, that, etc. The use of relative pronoun depends on the type of noun or pronoun present in the sentence. In the given question, the sentence that correctly uses a relative pronoun is option B as mentioned above as it uses "which" that modifies "that assignment" appropriately. The other three employs the incorrect pronoun usage relating to the subject of the sentence.