The correct option is D Designated rain shelters
A remarkable feature of the western war front was the extensive use of trench warfare, a type of combat in which opposing troops fight from trenches facing each other. These trenches ran parallel to each other, but were linked through communication trenches. They were dug in a zig-zag pattern to limit the reach of enemy fire. Food, ammunition, fresh troops, mail, and orders were delivered through these trenches. However, rain and snow in the trenches made them muddy, in turn making it difficult for soldiers to continue engaging in combat.