The correct answer is: C. Either I or II is implicit.
The statement is about the relationship between encouragement and performance. It does not say anything about whether or not all parents encourage their children, or whether or not parents will follow the principal’s advice. Therefore, either assumption I or assumption II could be true.
Assumption I is that some parents do not encourage their children. This is a possible assumption, because the statement does not say that all parents encourage their children.
Assumption II is that parents may follow the principal’s advice. This is also a possible assumption, because the statement does not say that parents will not follow the principal’s advice.
Therefore, either assumption I or assumption II could be true.