Final answer:
The main theme of the text is B. People can act in cruel ways when they hold power over others, as it reflects the complex nature of human interactions regarding morality and ethics highlighted throughout the text.
Step-by-step explanation:
From the given details, it's apparent that the main theme of the text revolves around the nuanced nature of human behavior and the complex moral and ethical questions that arise from our interactions. The proper selection that best describes this theme is B. People can act in cruel ways when they hold power over others. This choice is reflected in the text's discussion of negative human behaviors like aggression and the historical example of mass conformity during WWII. The text emphasizes the dual capacity for human beings to both commit great acts of kindness, such as helping during a pandemic, and great harm, like conforming to harmful group norms.
The other options do not as comprehensively capture the broad range of human interactions and moral complexities presented in the text. Instead of presenting a singular moral lesson, the text discusses multiple themes that explore the morality and ethical considerations that are paramount when humans live in a society together.