Which of the following is an example of an idiom?

An idiom is a phrase or expression that has a figurative meaning different from its literal meaning. The phrase "kick the bucket" exemplifies this concept well, as it does not literally mean to kick a physical bucket. Instead, it is a colloquial expression that signifies death. The figurative nature of this saying is what makes it an idiom; it relies on a cultural understanding of language rather than straightforward definitions of the individual words.

The other choices are more literal in their meanings. For example, "running up a hill" straightforwardly describes the action of jogging uphill without any figurative implications. "Sitting quietly" and "breaking the silence," while potentially having nuances beyond their literal meanings, do not reach the level of idiomatic expression because they can be understood directly and do not offer a meaning that is widely recognized as distinct from their literal interpretations. Thus, "kick the bucket" is a clear example of an idiom.