Final answer:
The correct word to use is 'morale' as it pertains to the troops' confidence and enthusiasm, which would be improved by receiving mail from home.
Step-by-step explanation:
Moral and morale may sound and look similar, but they have distinct meanings and uses. Moral is an adjective with the meaning of correct behavior; ethical. Morale is a noun describing the emotional state of a group or individual. The correct word for the sentence "The mail from home improved the troops' (moral, morale)" is b) Morale.
'Morale' refers to the confidence, enthusiasm, and discipline of a person or group at a particular time, especially in a challenging situation like war. In contrast, 'moral' pertains to the principles of right and wrong behavior. Thus, in the context of the troops receiving mail, it would be their morale that is improved, not their moral.