The correct answer is: All the business letters were answered yesterday. They were taken to the post office immediately.
The verb “answer” takes a direct object, so the answer must be a verb in the past tense that agrees with the subject “letters.” The verb “take” does not take a direct object, so it cannot be used in this sentence. The verbs “are answered” and “were taken” are both in the past tense, but they do not agree with the subject “letters.” The correct verb is “were answered,” which is the past tense of the verb “to answer.”
The sentence “All the business letters were answered yesterday” means that all of the business letters were responded to on the previous day. The sentence “They were taken to the post office immediately” means that the business letters were taken to the post office as soon as they were answered.