Choose the option that best completes the sentence. My uncle is not sure ____ the strange guests were.

To complete the sentence "My uncle is not sure ____ the strange guests were," we need to select a word that logically fits in the context of identifying or specifying something about the "strange guests."

1. **Which:** Typically used to choose between options or to specify a particular item or group from others. It does not fit well here because we are not choosing between a set of known options.

2. **Who:** Used to refer to the subject or inquiring about the identity of a person or people. This option fits well here since we are talking about people, the "strange guests," and trying to identify them. The sentence would be: "My uncle is not sure who the strange guests were."

3. **Whom:** Used to refer to the object of a verb or preposition and often follows a preposition. It would not fit here because the structure of the sentence requires a subject complement, not an object.

4. **What:** Generally used to ask questions about things, rather than people. Since we are talking about people, "strange guests," this option doesn't fit well.

Therefore, the best choice is **who**, as it properly addresses the question of identifying the strange guests, effectively completing the sentence as "My uncle is not sure who the strange guests were."