Answer: The right answers are number of stanzas, length of lines and rhyme scheme.
Explanation: Since the question is specifically asking for aspects of the poetic form, imagery should be excluded, since, unlike the number of stanzas, rhyme scheme and length of lines (or poetic meter), it does not refer to the physical structural elements of the poem. Those features shape the poem in a particular way, and that is why we refer to sonnets, odes, elegies, ballads, and so on. Imagery consists in using figurative language to make the text more persuasive and engaging (by going beyond the literal meaning of the words or describing a setting in a very visual way).