The correct answer is B. Ennui.
Ennui is a feeling of listlessness and boredom, arising from a lack of interest in one’s surroundings or in life in general. It is often associated with a feeling of futility or meaninglessness.
Fatigue is a state of physical or mental weariness or exhaustion. It can be caused by a number of factors, including lack of sleep, overwork, or illness.
Languor is a state of physical or mental lassitude or weariness. It is often associated with a feeling of weakness or fatigue.
Debility is a state of weakness or infirmity, especially one caused by disease or old age.
In the context of the question, ennui is the most appropriate answer because it is a feeling of mental weariness that
is specifically caused by a lack of occupation. Fatigue, languor, and debility are all general terms for physical or mental weariness, but they do not specifically refer to weariness caused by a lack of occupation.