What four types of trenches were used by the allies? this is a world war one question about the invasion of france by germans.

Front line trench: Soldiers stationed in the front-line trench were constantly on alert for enemy patrols and attacks. The front-line trench was also the start point of any "over the top" attack on an enemy. 

Support trench: The support trench help back-up forces to support the from-line trench in the emergency of an enemy attack or in the need of more soldiers for a friendly attack. They were also used to take supplies such as ammunition and food to soldiers on the front-line.

Reserve trench: Reserve trenches were even farther back than the support trenches. This was used to give soldiers a break from being on the front line. The reserve trench were still close enough in case an enemy attack got throughout the front trench and into the support trench.

Communication trench: The communication trenches ran the opposite direction as all the other trenches. These trenches aloud them to move soldiers and supplies from trench to trench without being exposed to the enemy. 

Im pretty sure the answer is d.) is more densely populated.