Answer:
B. By emphasizing their necessity in times of great struggle.
Step-by-step explanation:
A. By illustrating that most of their activity happened at night: This is taking is way too literal.
B. By emphasizing their necessity in times of great struggle: During war it is especially important to have a force that is reliable and able to defeat the opponent consistently. The more soldiers, the better. The question also includes that Washington was given a "small", but stable force.
C. By indicating that their training occurred at regular intervals: Perhaps this happened, but it is not a suitable response for what the idiom "Dark Hours" is referring to.
D. By explaining their need for periods of extreme secrecy: By saying dark hours, it is not telling the reader about their needs for things to stay private. My explanation for why the answer is B, stands.
Hope this helped, please let me know if this was correct!