Man’s humanity to his fellow man

which best explains the cultural relevance of this idea at the time when lord of the flies was published.

The Lord of the Flies was published in 1954, a few years after the end of World War II, and it was highly influenced by Golding's first-hand experience of the conflict - he had fought in it as a naval officer. In spite of the novel's pessimistic, yet sadly realistic, tone, its aim, I believe, is to stress the defects of humanity with the ultimate goal of encouraging it to improve and be kinder and more considerate. Hence the cultural relevance of this idea in the 1950s, when the world was still recovering from the largest and most terrible war in history, and it was necessary to remind its inhabitants of the importance of showing benevolence, compassion, disposition, and generosity, in other words, humanity, and of the dramatic consequences of not doing so, even through such bitter and harsh reminders as Golding's novel.  

For k-12 students the answer is C.) People easily and painfully recalled the terrible events of World War II.  

the rhetoric device being used to persuade the audience is pathos.