Final answer:
Macbeth's decision to kill the king is evidenced by his statement about preparing his body for the deed, found in the line 'I am settled, and bend up / Each corporal agent to this terrible feat.'
Step-by-step explanation:
The evidence from the text of Macbeth by William Shakespeare that shows Macbeth has finally decided to kill the king is found in option C: I am settled, and bend up / Each corporal agent to this terrible feat. This line indicates Macbeth's resolution to commit the murder, as he prepares himself both mentally and physically to carry out the act. He speaks of rallying every part of his body (corporal agent) to perform the treacherous action (terrible feat), which marks his decisive moment.