**Core Concept**
The QRS complex represents the depolarization of the ventricles, and its voltage is influenced by the size and orientation of the cardiac silhouette. In electrocardiography, the QRS voltage is measured in millivolts (mV) and is typically highest in lead II due to the orientation of the heart in the chest.

**Why the Correct Answer is Right**
In a normal electrocardiogram, the QRS voltage is highest in lead II due to the perpendicular alignment of the QRS complex with the chest wall. This is because the QRS complex is primarily generated by the depolarization of the ventricles, which are oriented in such a way that their electrical activity is maximally recorded in lead II. The QRS voltage in lead II is typically greater than in leads I and III.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.5 mV - This is the QRS voltage in lead I, not lead II, and is the lowest of the three leads.
**Option B:** 1.5 mV - This is the QRS voltage in lead III, not lead II, and is lower than the expected value in lead II.
**Option C:** 2.5 mV - This is an arbitrary value that does not reflect the expected relationship between the QRS voltages in the different leads.

**Clinical Pearl / High-Yield Fact**
When interpreting electrocardiograms, it's essential to consider the orientation of the cardiac silhouette and the expected voltage in each lead. In general, the QRS voltage is highest in lead II, followed by lead III, and then lead I.

**Correct Answer: C. 2.0 mV.**