Final answer:
The reference or ground electrode in a 12-lead ECG is the left leg electrode, which stabilizes the baseline of the ECG trace and reduces electrical interference.
Step-by-step explanation:
In a 12-lead ECG (electrocardiogram), four electrodes are placed on the limbs to collect electrical signals from the heart. Each limb electrode serves a different purpose, with one designated as the ground or reference electrode.
The correct answer to the question of which limb sensor is the reference or ground electrode is C.
Left leg. The left leg electrode acts as the electrical ground, and it is used as a reference point for the other electrodes in order to reduce electrical interference and stabilize the baseline of the ECG trace.