Which term refers to a gentle reproof or warning?

The term "admonition" refers to a gentle reproof or warning, making it the most suitable choice in this context. It implies an expression of disapproval or caution without being overly harsh or critical. Admonitions are often given with the intention of guiding someone towards better behavior or decisions, rather than simply criticizing them.

In contrast, affirmation relates to the act of confirming or validating something, which doesn’t convey the notion of warning or reproach. Advice also suggests offering guidance or suggestions, but it lacks the specific connotation of a gentle reprimand. Accusation implies blaming or charging someone with wrongdoing, which is much stronger and more confrontational than a gentle reproof. Therefore, "admonition" is the term that accurately describes a gentle reproof or warning.