She was sceptical about the safety of the new drug.

Answer: Option C

Sceptical : not easily convinced; having doubts or reservations.
Sanguine : optimistic or positive, especially in an apparently bad or difficult situation.
Hopeful : feeling or inspiring optimism about a future event.
Certain : able to be firmly relied on to happen or be the case.
Doubtful : feeling uncertain about something.

Antonym of Sceptical is Certain