Mary had a little lamb whose fleece was white as snow.

A. Analogy

B. Idiom

C. Metaphor

D. Simile

D. simile. It uses 'as'. A simile uses like or as to compare something

I think the correct answer from the choices listed above is option D. The figure of speech being used here is a simile. The use of the word "as" shows that a simile is being used to compare the quality of the fleece to snow. Hope this answers the question. Have a nice day.