The poetry element that affects the poem's sound is rhythm, that is, your answer would be D. Rhythm is a literary device that refers to the patterned reocurrence of certain language features (specially sound) in a regular way. In English, there are five important rhythms: lamb, trochee, spondee, dactyl and anapest. They are different patterns of stressed and unstressed syllables.
Example: Shall I compare thee to a summer's day?
This is an example of the type of rhythm known as lamb. It consists of two syllables: the first one is not stressed while the second one is stressed.