The main idea of a poem is also known as the: a. meterb. soundc. subjectd. type

The answer is C: subject.

Poetry, like any other form of literature, usually treats a subject matter or topic. A subject is the underlying idea that is thematically explored in a work of poetry, but the subject should not be confused with the theme of a poem. In colloquial terms, a subject is the topic elaborated upon in the poem, and the theme is, most commonly, the poet´s personal perspective on the subject matter.    

what is the context here? we need to know what story you're talking about : )