The surgeon started to perform the operation, but the patient was not yet completely under anesthesia.

The phrase "started to" indicates the beginning of an action but does not specify the appropriateness or timing of that action in relation to the patient's anesthesia. While it acknowledges that an action commenced, it does not directly reflect the issue of the operation being performed prematurely.

The phrase "was not" serves as a negation of the patient's state of being under anesthesia. However, it does not directly address the implications of the surgeon's actions. This choice merely states a fact about the patient's condition rather than the active decision-making process involved in the surgery.

The phrase "completely under" describes the desired state of the patient regarding anesthesia. While it highlights what is needed for the surgery to proceed safely, it does not emphasize the surgeon's action of starting the operation. This choice focuses on the patient rather than the surgeon's decision, which is the central issue of the sentence.