The correct answer is C. Either I or II is implicit.
Assumption I is not implicit. The statement does not say that only writers refer to the dictionary. It is possible that other people, such as students, teachers, or editors, also refer to the dictionary.
Assumption II is also not implicit. The statement does not say that all writers, good or bad, refer to the dictionary. It is possible that some writers do not refer to the dictionary, even if they are good writers.
However, both assumptions are possible explanations for why A would advise B to learn to refer to the dictionary. Therefore, either assumption I or assumption II is implicit in the statement.