Explanation:

Present Continuous Tense (is studying):
- The correct form would be "She is studying for her exams on weekends."
- This form is used to talk about actions that are happening at the moment of speaking or around the current time.
- However, in this context, the sentence is referring to a habitual action (studying for exams on weekends), so the present continuous tense would not be the most suitable choice.

Future Continuous Tense (will be studying):
- The correct form would be "She will be studying for her exams on weekends."
- This form is used to talk about actions that will be in progress at a specific future time.
- It implies that she has a plan to study for her exams on weekends in the future.
- This choice is the most appropriate as it captures the idea of a future action that will be ongoing.

Past Perfect Tense (had studied):
- The correct form would be "She had studied for her exams on weekends."
- This form is used to talk about an action that was completed before another past action.
- It implies that she studied for her exams on weekends before some other past event.
- This choice is incorrect as it does not convey the intended meaning of a future action of studying for exams on weekends.