The correct answer is C. sober.
A bibulous person is one who drinks a lot of alcohol. So, if there was not a single bibulous adventurer in the expedition, it means that all of the adventurers were sober.
The other options are not correct because they do not describe the adventurers in the way that the question is asking.
Fearful: An adventurer is someone who is willing to take risks and explore new places. It is unlikely that an adventurer would be fearful.
Cowardly: A coward is someone who is afraid to take risks. It is unlikely that an adventurer would be cowardly.
Unenergetic: An adventurer is someone who is full of energy and excitement. It is unlikely that an adventurer would be unenergetic.