Answer: D. Escape
Step-by-step explanation:
The word that could be used to describe the central idea of the sentence is "escape"
The sentence is trying to explain how they always tried to escape out of there, out of the steerage, out of the babble voices, out of the heat and stench and get on the main deck. They were permitted to stay on the main deck for a while but they were constantly chased away.
This explains how they were trying to escape discomfort to get comfort.