The correct answer is A. Cotton-like clouds.
This is a rather descriptive choice that is used to bring the image of cotton to your mind and then use that image of cotton to create a new image of the clouds. Unlike the soldiers, it not only describes the looks, but also the feeling, the fluffiness of cotton.
Not quite, in that case lecture is a noun.
If the sentence read: The students were being lectured
then that would be a verb
Gloomy
The author's tone is is gloomy as it is discussing death.