Rewrite "He studied hard" as a suggestion.

The suggestion to rewrite "He studied hard" emphasizes an affirmative recommendation regarding his study habits. By using "He should study hard," the suggestion conveys that it is advisable or beneficial for him to engage in hard study. This form presents a strong recommendation based on a belief that studying hard is the right course of action for him, indicating that improvement or success can result from such effort.

In contrast, options like "He might study hard" and "He can study hard" imply possibility or capability but lack the strong recommendation present in the correct choice. "He would study hard" also suggests a conditional or hypothetical situation, which doesn't effectively convey a direct suggestion for the action. Therefore, the phrasing in the correct answer directly aligns with the intent of providing advice or guidance.