Concept:The verb "dictate" requires the preposition "to" (not "with") when specifying the person who receives the dictation.
Explanation:The sentence describes actions: opening letters, reading them, and then dictating answers.
Part (c) says "he dictated answers with them".
The word "dictate" means to say words aloud for someone else to write down.
The correct preposition after "dictate" is "to", not "with".
For example: "The manager dictated a letter to his secretary."
Therefore, "with them" is incorrect; it should be "to them".
The error lies in part (c).
Answer:(c)