Answer:
D. Idiom
Explanation:
The phrase "piece of cake" is an idiom.
It's not a hyperbole because we're not exaggerating, it's not alliteration because the letters are not repeating, it's not personification because you are not giving a non living thing human characteristics.
It is not a simile because you're not using like or as, it's not a metaphor, it's not an onomatopoeia, or sensory imagery.
