They are cautiously optimistic about the effects of the new medicine.

Answer: Option A

Carefully : in a way that deliberately avoids harm or errors; cautiously.
Genuinely : in a truthful way.
Secretly : in a secret way; without others knowing.
Curiously : in a way that shows eagerness to know or learn something.
Cautiously : in a way that deliberately avoids potential problems or dangers.

Synonym of Cautiously is Carefully