Final answer:
The modal verb 'will' is appropriately used in the sentence to express the possibility of a future event, which is why the correct answer is (a) Because the sentence is about the remote future.
Step-by-step explanation:
The use of 'will' in the sentence 'Perhaps she will pass her exam' is appropriate because it is used to express the possibility or likelihood of an event happening in the future. The correct answer is therefore a) Because the sentence is about the remote future. The modal verb 'will' is not indicating a spontaneous decision nor simply about the act of passing the exam; it is relating to a future event that is uncertain.