The correct answer is: is operating.
The sentence “We can’t disturb him now. He ____” is in the present continuous tense. This tense is used to talk about actions that are happening at the moment of speaking. In this case, the speaker is saying that they cannot disturb the person because they are currently operating on someone.
The other options are not correct because they do not fit the context of the sentence. Option A, “operate”, is in the simple present tense. This tense is used to talk about general truths or habits. Option B, “will operate”, is in the future tense. This tense is used to talk about actions that will happen in the future. Option C, “has operated”, is in the simple past tense. This tense is used to talk about actions that happened in the past.
Therefore, the correct answer is “is operating”.