Explanation:
> "I started writing this test at 8:00 a.m. It is 10:00 a.m now. By 11:00 a.m. I ____ it for three hours."

The phrase "By 11:00 a.m." suggests that we need a future perfect continuous tense to show an action that will have been happening for a period of time up to a point in the future.

Now, let's check the options:

- A. shall be writing — Future continuous; it describes what will be happening at a future time, but not the *duration*. Incorrect.
- B. have been writing — Present perfect continuous; it refers to now, but we are talking about future. Incorrect.
- C. shall have been writing — Future perfect continuous; exactly what we need: duration of action up to a point in future. Correct.
- D. am writing — Present continuous; does not fit the time frame. Incorrect.
- E. was writing — Past continuous; wrong tense. Incorrect.