## Core Concept
The question assesses the developmental milestone related to gender identity in children. Understanding the age at which children develop the concept of gender and can identify themselves or others as male or female is crucial.

## Why the Correct Answer is Right
Children typically develop the understanding of gender identity between 2 to 3 years of age. This is a part of their cognitive and social development. By around 2 years old, children start to have a basic understanding of gender but might not fully comprehend the permanence of gender until they are a bit older. The correct answer reflects this developmental milestone.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it suggests an age that is too early for most children to demonstrate an understanding of gender.
- **Option B:** This option might seem plausible but does not accurately reflect the age by which most children can tell their gender.
- **Option D:** This option suggests an age that is older than necessary for children to typically understand and express their gender.

## Clinical Pearl / High-Yield Fact
A key point to remember is that by the age of 2 to 3 years, children often begin to use gender labels and can identify themselves and others as male or female, though their understanding of gender roles and stereotypes may evolve further over time.

**Correct Answer: C. 2-3 years.**