My uncle is not sure … the strange guests were.

Analyze the question

The sentence is: "My uncle is not sure __ the strange guests were." We need the word that correctly introduces the clause "the strange guests were _."

Explain the concept

The clause after "sure" is a noun clause (an indirect question). In the original question form, it would be:

"Who were the strange guests?"

The verb "were" is a linking verb. It connects the subject "the strange guests" to a complement. Because "who" asks about a person's identity, it fits when we want to know the identity of people.

Check each option

• A. which: Used when choosing from a limited, known set ("which of the guests"). Nothing in the sentence suggests a limited set, so it is not the best fit.
• B. who: Asks about the identity of people. "...not sure who the strange guests were" means he doesn't know their identities. This fits.
• C. whom: The object form, used after prepositions or as the object of a verb. Here "were" is a linking verb, so no object is needed. Also, "whom the guests were" is not correct grammar.
• D. what: Asks about things or about nature and description. It would be odd for people, though "what" can occasionally be used to ask about a person's role or occupation ("what they were"). Here, the natural meaning is identity.

Solution

Rewriting the sentence: "My uncle is not sure who the strange guests were."

This is correct and natural.

Answer: B. who

Exam tip

When a linking verb (is, are, was, were) follows the blank, the missing word is usually a complement, not an object, so use who rather than whom. A quick test: replace the blank with "they." If "they" works ("the guests were they"), the subject form "who" is appropriate. If "them" would work, use "whom."