**Core Concept**
Normal QRS axis in electrocardiography (ECG) refers to the normal orientation of the electrical activity of the heart. The QRS axis is measured in degrees and represents the direction of the electrical impulse as it travels through the ventricles. A normal QRS axis is between -30° and +100°.

**Why the Correct Answer is Right**
The correct answer is based on the normal orientation of the heart's electrical activity. The QRS axis is determined by the direction of the electrical impulse as it travels through the ventricles. A normal QRS axis is between -30° and +100°, which corresponds to the normal anatomical position of the heart. This is due to the normal orientation of the ventricular conduction system, which includes the bundle branches and the Purkinje fibers. The QRS axis is influenced by the position of the heart in the chest and the direction of the electrical impulse as it travels through the ventricles.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a QRS axis between 0° and -30° is considered left axis deviation, which is abnormal.
**Option B:** This option is incorrect because a QRS axis between 100° and 180° is considered right axis deviation, which is abnormal.
**Option C:** This option is incorrect because a QRS axis between 180° and 270° is considered extreme right axis deviation, which is abnormal.

**Clinical Pearl / High-Yield Fact**
It's essential to note that a normal QRS axis is between -30° and +100°, and any deviation from this range may indicate underlying cardiac pathology. Additionally, a QRS axis of +110° or more is often considered abnormal.

**Correct Answer: C. +90°**