**Core Concept**
The size of the laryngeal mask airway (LMA) is determined by the internal diameter of the airway, which is a critical factor in ensuring adequate ventilation and preventing complications in normal adults.

**Why the Correct Answer is Right**
The size of the LMA is directly correlated with the internal diameter of the airway. A larger internal diameter is associated with better ventilation and reduced risk of complications such as airway obstruction and gastric insufflation. The correct size of the LMA for normal adults is typically 4-5 mm in internal diameter, which corresponds to a size 4 or 5 LMA.

**Why Each Wrong Option is Incorrect**
**Option A:** This size is too small for normal adults and may lead to inadequate ventilation.
**Option B:** This size is too large for normal adults and may cause airway obstruction or gastric insufflation.
**Option C:** This size is not a standard size for LMA and may not be available in most medical settings.

**Clinical Pearl / High-Yield Fact**
When selecting an LMA, it is essential to choose a size that is appropriate for the patient's internal diameter to ensure effective ventilation and prevent complications.

**Correct Answer: C. 4-5 mm**