Answer: Remembering things.
Step-by-step explanation:
Of Mice and Men is a novella that's written by John Steinbeck. The novella narrates the experience of two displaced migrant whom were ranch workers who were looking for jobs in California during the Great Depression.
The men searching for jobs were Lennie Small and George Milton. Based on the dialogue in the reading excerpt, Lennie struggles mainly with remembering things.
The correct option is D.