. . . . . . a doctor . . . . . . in time, the patient can be saved. A. Had , arrived B. If , arrives C. Do , arrive D. Will , arrive

The correct answer is: If a doctor arrives in time, the patient can be saved.

The other options are incorrect because they do not express the certainty that the patient can be saved if a doctor arrives in time.

• Option A, Had a doctor arrived in time, the patient can be saved, is in the past tense. This suggests that the doctor did not arrive in time, and so the patient could not be saved.
• Option B, If a doctor arrives in time, the patient can be saved, is in the conditional tense. This suggests that it is possible that the doctor will arrive in time, but it is not certain.
• Option C, Do a doctor arrive in time, the patient can be saved, is in the imperative tense. This suggests that someone should make sure that a doctor arrives in time, but it does not say that the doctor will actually arrive in time.
• Option D, Will a doctor arrive in time, the patient can be saved, is in the future tense. This suggests that it is certain that the doctor will arrive in time, but this is not necessarily the case.

In conclusion, the correct answer is: If a doctor arrives in time, the patient can be saved.