Choose the phrase that best completes the dinner host's anxious thought.

I'm not entirely sure _____ lactose intolerant.

The correct answer is if the Martian is.

For indirect yes/no questions, we must use "if" or "whether" to connect the clauses, followed by standard Subject-Verb word order. "Is the Martian lactose intolerant?" becomes "...if the Martian is lactose intolerant."