The correct answer is option 'C': pulled out the last drawer.

Explanation:
The given question is about a narrator who found a shallow space below a secret drawer. Here, we need to identify the event or action that led the narrator to discover this space. Let's analyze each option to understand why it is or is not the correct answer.

Option A: pulled out the first drawer
If the narrator found the shallow space below a secret drawer by pulling out the first drawer, then it implies that the secret drawer was the first one in the sequence. However, the phrase "below a secret drawer" suggests that the secret drawer was located somewhere in the middle or towards the end of the sequence. Hence, option A cannot be the correct answer.

Option B: began to mend the desk
There is no direct connection between mending the desk and discovering a secret compartment. Although it is possible that the narrator might have stumbled upon the compartment while repairing the desk, we cannot assume it from the given information. Therefore, option B is not the correct answer.

Option C: pulled out the last drawer
This option is the correct answer because it states that the narrator found the shallow space below a secret drawer when they pulled out the last drawer. It makes sense because the secret drawer could be hidden at the bottom of the sequence, and pulling out the last drawer would reveal it. Therefore, option C is the correct answer.

Option D: had finished working
This option does not provide any context or information about what the narrator was working on. It is unclear whether they were working on the desk or something else. Without this information, we cannot determine if this option is the correct answer. Hence, option D is not the correct answer.

In conclusion, the correct answer is option C: pulled out the last drawer, because it is the only option that logically connects with the given information to lead the narrator to find the shallow space below a secret drawer.

Pulled out the last drawer