HUMS normally stands for Health and Usage Monitoring System. However the question calls it Helicopter Usage Monitoring System - ie no Health.
Perhaps this is a mistake on the part of the question-setter, but on the other hand, looking at the 3 answers, 2 of them relate to the "Health" aspect and one relates to the "Usage" aspect of HUMS - so perhaps the Health is deliberately left out and the correct answer is b.
In other words, if the question is meant as it is exactly written, b is the answer. However it does seem a silly question to ask because everyone in the civil industry will tell you that it is answers a and c - and "Health" that is the main reason and benefit of having HUMS fitted. I seem to recall that is was the military that were pushing for the Usage aspect to be designed into HUMS - presumably because their existing methods of tracking component lives were inadequate at the time. Perhaps the questioner has a military background?
If the question had read "The Health & Usage Monitoring System (HUMS) provides " then the correct answer would certainly have been a - or at least that is the main justification for having HUMS in the eyes of civilian operators and the CAA.