**Core Concept**
Most children develop an awareness of their own sex and can identify their gender by the age of 3 years. This milestone is a significant part of cognitive development and social learning, where children begin to understand and differentiate between masculine and feminine characteristics.

**Why the Correct Answer is Right**
By the age of 3 years, most children have a clear understanding of their own sex and can accurately identify themselves as male or female. This development is influenced by a combination of cognitive, social, and environmental factors, including parental guidance, social interactions, and cultural influences. Children at this stage typically demonstrate an understanding of basic sex roles and begin to exhibit gender-typed behaviors. The emergence of gender awareness is closely linked to the development of language skills, as children learn to use sex-specific pronouns and labels.

**Why Each Wrong Option is Incorrect**
**Option A:** 1 year is too early for children to develop a clear understanding of their own sex, as they are still in the process of learning language and social skills.
**Option B:** 2 years is still within the range of normal variability, but most children do not demonstrate a clear understanding of their own sex until around 3 years of age.
**Option D:** 4 years is an age at which children typically have a well-established understanding of their own sex and are able to accurately identify themselves as male or female.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the development of gender awareness can vary significantly among children, and some may take longer to reach this milestone. Pediatricians and caregivers should be aware of these individual differences and provide supportive guidance to help children develop a positive sense of self and their own sex.

**✓ Correct Answer: C. 3 years**