The correct answer is D: he did not have time to be tired.
Trenches were long, narrow ditches dug into the ground where the soldiers lived.
They were always muddy, uncomfortable, unhygienic with overflowing toilets. The soldiers living in them were busy filling sandbags, putting them up as walls, organizing and delivering supplies and protecting themselves from their enemies.
Soldiers only got to sleep in the afternoon during daylight and at night for an hour at a time. During rest time they played cards or wrote letters.
They did not have time to be tired.