A heart of stone in the lines below is an example of?

This question is based on General Literary Appreciation.

‘I have said too much unto a heart of stone,
And laid my honour too unchary on it’,
There’s something in me that reproves my fault,
But such a headstrong potent fault it is
That it but mocks reproof.’.

William Shakespeare: Twelfth Night

A. litotes
B. metonymy
C. assonance
D. metaphor

Correct Answer:

Option D – metaphor