Ответ:
a. have studied;
d. study;
8. I am studying right now.
9. I have been studying for two hours.
10. I was studying Chapter One.
11. I had studied Chapter One, before I began to study Chapter Two.
12. I had been studying for hours, when Mother came.
Объяснение:
В 10 примере я указал прошедшее длительное. Но, по сути из оставшихся вариантов можешь использовать любой. Т.к. временные маркеры не указаны.