。
Study.
Literal
Study (polite command).
The minimal imperative form: just 勉強する + ~なさい. A one-word parental or teacherly instruction, firm but not harsh. Worth comparing with the rougher plain imperative 勉強しろ (rarely shown in textbooks but heard in real life from frustrated parents or elder siblings) and the prohibition 勉強するな ('don't study'), which uses the negative-imperative ~な.