The correct answer is: Only assumption II is implicit.
The statement is an announcement that advises passengers to fasten their seat belts while seated in the trolley of the ropeway. This suggests that passengers might not fasten their seat belts unless they are advised to do so. This is supported by the fact that the announcement is made in the passengers’ own interest. Therefore, assumption II is implicit.
Assumption I is not implicit. The statement does not say that people are always careful about their own safety. It is possible that some people are careful about their own safety, while others are not. The statement only advises passengers to fasten their seat belts, regardless of whether they are careful about their own safety or not.