The correct option is
B By operating upon him immediately.
The given passage mentions that the young doctor decided to operate on the patient immediately. It also mentions that the prompt decision of the young doctor (to operate) saved the life of his patient. Hence, option B is the correct answer.
The passage does not talk about the young doctor giving medicines to the patient. Hence, option A is incorrect.
Option C: The young doctor consulted two other doctors to find out if an operation should be performed. However, he did not take their advice that the patient's condition should be observed for a few days. Hence, the young doctor did not save the life of his patient by consulting other doctors. Option C is incorrect.
Option D: Before deciding to operate on the patient, the young doctor examined his patient once again. This tells us that he was responsible and thought through the patient's condition before taking his decision. Hence, option D is incorrect.