The error lies in segment (C). It contains two issues:
% Option
(A) Incorrect verb usage:
The verb "laid" is the past tense of \textit{"lay"} (meaning to put something down). However, in this sentence, the intended meaning is that the subject (I) reclined or rested — which requires the verb "lay" (past tense of "lie").
So the correct verb is: "lay", not "laid".
% Option
(B) Incorrect noun usage:
The word "coach" is wrong in this context. "Coach" refers to a bus or a sports trainer. The correct word for a piece of furniture one can lie on is: "couch".
Corrected sentence:
"After returning from the post office, I lay down on the couch to rest."