Which of the following is an example of an idiom?

An idiom is a phrase or expression whose meaning cannot be understood from the literal interpretation of its individual words. The phrase "to bite the bullet" is a commonly recognized idiom that conveys the idea of facing a difficult or unpleasant situation with courage and resolve. This figurative language evokes a vivid image of enduring pain or hardship, suggesting that one should accept a challenging circumstance rather than avoid it.

In contrast, the other phrases—like "to run a marathon," "to jump over a hurdle," and "to walk in the park"—are more literal in meaning. Each of these can be understood directly based on the definitions of the individual words without any figurative context. Thus, they do not fit the criteria of an idiom, which relies on a metaphorical understanding that extends beyond the specific words used.