**Core Concept**
The average length of a full-term child is an essential parameter in pediatric medicine, used to assess fetal growth and development during pregnancy. It is also a critical factor in determining the gestational age and identifying potential growth restriction or other complications.

**Why the Correct Answer is Right**
The average length of a full-term child is approximately 50 cm (20 inches) at birth. This measurement is influenced by the length of the fetus's trunk, limbs, and head, which increase in proportion to the overall growth of the fetus. The correct answer reflects the average length of a full-term child, which is a widely accepted standard in pediatric medicine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the average length of a full-term child is typically not less than 45 cm (18 inches), which would be considered growth restriction.
**Option B:** This option is incorrect because the average length of a full-term child is not greater than 55 cm (22 inches), which would be considered macrosomia.
**Option C:** This option is incorrect because the average length of a full-term child is not typically less than 48 cm (19 inches), which would be considered growth restriction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the average length of a full-term child can vary depending on the population and other factors, but 50 cm (20 inches) is a widely accepted standard.

**Correct Answer:** C. 50 cm