Final answer:
In 'Of Mice and Men', Slim is the character who understands why George shoots Lennie, recognizing it as an act of mercy to prevent a worse fate. The correct answer is option B) Slim.
Step-by-step explanation:
The character who knows and understands the reasons why George shoots Lennie in 'Of Mice and Men' is Slim. Slim is depicted as a character with a deep understanding of the people on the ranch and is sensitive to the complexities of human relationships and emotions.
He comprehends that George's decision to kill Lennie was to prevent a worse fate at the hands of a mob led by Curley. Unlike the others, Slim understands the sacrifice George made out of love and mercy for Lennie, having seen the turmoil George was in over the decision. After the shooting, Slim is the one who consoles George, indicating his understanding of the gravity of the situation.