Question

What evidence from the text supports the idea that Lady Macbeth believes here husband is too weak to kill the king?

Answer 1

Your answer is C because Macbeth was not cruel enough to kill the king for selfish reasons but Lady Macbeth obviously shows no remorse 

Answer 2

C. I do fear thy nature ( Basically I'm not confident that your personality...) Is too full of the milk of human kindness (is too nice, caring, sympathetic) to catch the nearest way (to kill enemies or do what is necessary, aka kill the king, so that you can become king)

So it means she is afraid that her husband is too much of a nice guy to kill anyone.