This question tests the understanding of the past perfect tense, which indicates an action completed before another action in the past. - **A. She is finishing the book last week.:** This uses the present continuous tense with a past time marker which is grammatically incorrect. - **B. They will have arrived by the time we leave.:** This sentence is using the future perfect tense, which does not fit the requirements. - **C. He had eaten breakfast before he left for work.:** This sentence correctly uses the past perfect tense 'had eaten' to indicate that eating breakfast happened before he left for work, which is an action also in the past. This is the correct option. - **D. They are playing basketball now.:** This is in the present continuous tense and doesn't involve a past action. The key to identifying the correct answer is understanding that the past perfect shows that one past action occurred before another.