The simple and rhythmic sounds of Henry Wadsworth Longfellow's "The Rainy Day" is a product of the poem's C.) Rhyme and Meter.
Meter refers to the pattern of the beats or the sound of the poem. Rhyme refers to the system or scheme used throughout the poem characterized by words used in every line.
Therefore, the rhyme is responsible for the simplicity of the poem because the rhyme scheme followed unites all the lines while the meter is responsible for the rhythm that dictates the beat or sound of the poem.