Choose the option that best completes the sentence. I can't lend him my books. I ______ know him.

The sentence "I can't lend him my books. I ______ know him." is meant to express the idea that the speaker does not have enough familiarity or trust with the person in question to lend their books.

The best choice to complete the sentence is "scarcely". This is because "scarcely" means almost not or barely, indicating a very low level of knowing or familiarity. Thus, it implies that the speaker knows the person just a little, which aligns with the reason they are hesitant to lend their books.

To clarify the other options:
- "Often" suggests a frequent occurrence, which is not fitting because the sentence conveys a lack of familiarity.

- "Seldom" means not often, and while it may indicate infrequency, it is not as strong as 'scarcely' in expressing minimal familiarity.

- "Rarely" is also more infrequent than "seldom," yet it doesn't convey the minimal familiarity as effectively as "scarcely."

Therefore, "scarcely" is the best word to complete this sentence because it most accurately communicates a low level of acquaintance or trust.