**Core Concept**
Subglottic stenosis refers to the narrowing of the subglottic region, which is the area below the vocal cords and above the trachea. In term infants, subglottic stenosis is often diagnosed based on the subglottic diameter, which is a critical parameter in assessing airway patency.

**Why the Correct Answer is Right**
The correct answer is based on the American Academy of Pediatrics (AAP) guidelines, which define subglottic stenosis in term infants as a subglottic diameter of less than 3.5 mm. This is because a subglottic diameter of less than 3.5 mm is associated with significant airway obstruction and may require surgical intervention. The subglottic diameter is measured using a laryngoscope and is an essential parameter in assessing the airway in term infants.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a standard definition of subglottic stenosis in term infants.
**Option B:** This option is incorrect because it is not a widely accepted definition of subglottic stenosis in term infants.
**Option D:** This option is incorrect because it is not a standard definition of subglottic stenosis in term infants.

**Clinical Pearl / High-Yield Fact**
In term infants, subglottic stenosis is often associated with respiratory distress, stridor, and feeding difficulties. A subglottic diameter of less than 3.5 mm is a critical parameter in diagnosing subglottic stenosis, and prompt surgical intervention may be necessary to prevent long-term airway complications.

**Correct Answer:** C. 3.5 mm.