Explanation:

He spoke Confidently about his prospects.
Conscientiously: in a thorough and responsible way.
Confidently: in a self-assured way that expresses faith in oneself or one's abilities or qualities.
Consciously: in a deliberate and intentional way.
Confidentially: in a way that is intended to be private or secret; privately.
The best suitable adverb for the above given sentence is “Confidently”.
Hence, option 3 is the corerct answer.