What defines a "sonnet"?

A sonnet is specifically defined as a fourteen-line poem that is typically written in iambic pentameter. This form of poetic structure has a distinct rhythm, where each line generally consists of ten syllables arranged in five pairs of unstressed and stressed syllables. The sonnet conventionally explores themes of love, beauty, time, and nature, often employing a specific rhyme scheme.

The other options represent different literary forms. A short story with a moral lesson refers to a narrative prose composition, while free verse poems do not have a fixed meter or rhyme scheme. A lengthy narrative poem would describe an epic, which differs significantly from the concise and structured nature of a sonnet. Thus, the defining characteristic of a sonnet is its specific length and meter, making the correct answer focused on the structure and form of this particular type of poetry.