The correct option to fill the gap in the sentence "I was able to .... Olu my book because he promised to return it the following day" is C. lend. Step-by-Step Explanation:

1. Understanding the Context: The sentence indicates that the speaker is allowing someone named Olu to use their book with the expectation that Olu will return it the next day. This context is crucial for determining the correct verb to use.
2. Analyzing the Options:
3. A. borrow: This means to take something from someone with the intention of returning it later. If the sentence used "borrow," it would imply that Olu is taking the book from the speaker, which does not fit the structure of the sentence. The speaker is the one giving the book, not taking it.
4. B. give: This means to transfer possession of something to someone else. While it might seem plausible, "give" does not convey the idea of temporary use with the expectation of return. The sentence specifically mentions that Olu promised to return the book, which aligns more with lending than giving.
5. C. lend: This means to allow someone to use something temporarily with the expectation that it will be returned. This option fits perfectly in the context of the sentence, as it indicates that the speaker is allowing Olu to use the book with the promise of its return.
6. D. lease: This term typically refers to a formal agreement to rent property or equipment for a specified period. It is not appropriate in this context, as it implies a business transaction rather than a casual lending of a book.
7. Why C is Correct: The verb "lend" is the most suitable choice because it captures the essence of the action being described: the speaker is allowing Olu to use the book temporarily, with the understanding that it will be returned. This aligns perfectly with the promise mentioned in the sentence.
8. Why the Other Options are Incorrect:
9. A. borrow: Incorrect because it suggests that Olu is taking the book, which is not what the speaker is doing.
10. B. give: While it could fit grammatically, it does not convey the temporary nature of the action or the expectation of return.
11. D. lease: Incorrect as it implies a formal agreement and is not suitable for the context of lending a book.

Revision Summary:

• Lend means to allow temporary use of something with the expectation of return.
• Borrow means to take something with the intention of returning it, which does not fit the speaker's action.
• Give implies a permanent transfer of ownership, not temporary use.
• Lease refers to a formal rental agreement, inappropriate for casual lending situations.

By understanding the nuances of these verbs, you can choose the correct option based on the context of the sentence.