Fill in the blank with the most appropriate word. She ________ the prize because she wrote the best composition.

The correct answer is A. won.

The sentence is in the past tense, so the verb must also be in the past tense. The other options are not in the past tense.

• B. to win is in the infinitive form, which is not used in this context.
• C. is winning is in the present continuous tense, which is not used to describe a completed action.
• D. wins is in the simple present tense, which is not used to describe a past action.

Therefore, the only option that is grammatically correct and makes sense in the context of the sentence is A. won.