The professor raised his eyebrows, showing how doubtfully he was receiving my explanation.
Yes, it is an adverb. it is the adverb form of doubtful (doubting).
Doubtfully is the correct spelling.
The adjective form of doubtfully is doubtful.
Doubtfully
Doubtfully.
Doubtfully
doubtfully, uncertainly
if you say you will become a millionaire i will doubtfully say no. it means i dont think something will happen or i dont think you will do something.
Suspiciously, distrustfully, doubtfully.
Antonyms: doubtfully, gloomily, hopelessly
The opposite could be unsure, or uncertain.
Doubtfully. They have to be the same.