The correct answer is: Only III is implicit.
The statement says that Mr. A tells his clerk to give a packet to Mr. X at his residence and return immediately. It also says that in case the clerk is likely to be late, he should inform Mr. A.
Assumption I is that the clerk never informs about his late coming. This assumption is not implicit in the statement. The statement only says that the clerk should inform Mr. A if he is likely to be late. It does not say that the clerk never informs about his late coming.
Assumption II is that the clerk may not obey Mr. X instructions. This assumption is also not implicit in the statement. The statement only says that Mr. A tells his clerk to give a packet to Mr. X at his residence and return immediately. It does not say that the clerk may not obey Mr. X instructions.
Assumption III is that the clerk may not inform his late coming unless instructed. This assumption is implicit in the statement. The statement says that the clerk should inform Mr. A if he is likely to be late. It does not say that the clerk will inform Mr. A even if he is not likely to be late. It is possible that the clerk will only inform Mr. A if he is instructed to do so.
Therefore, the only implicit assumption is III.