Fill in the blank with the most appropriate word.
Who ________ next to you now?

In the given sentence "Who is sitting next to you now?" the most appropriate word to fill in the blank is "is sitting."

The sentence is asking about the current action of someone sitting next to the person being addressed. The continuous tense is used to describe an action that is happening at the moment of speaking. Therefore, the correct form is "is sitting."

The other options do not use the correct verb form for the given sentence. Option A, "sits," is in the simple present tense, which does not convey the idea of the action happening at the moment. Option C, "sit," is the base form of the verb and is not suitable for this context. Option D, "sitting," is a present participle form and does not fit grammatically as the main verb in this question.

Therefore, the correct answer is "B. is sitting."