Okay, let's tackle this question. The question is asking at what age a child knows their sex. Hmm, I remember that gender identity development is a key topic in child psychology. Let me recall the stages.

First, the core concept here is gender identity development in children. I think there are different stages. Like, by the age of 2-3, they might start to recognize their sex, but it's more consistent around 3-4 years old. Wait, I need to be precise.

The correct answer is probably around 3 years old. Because I remember reading that by age 3, most children can consistently identify their own gender. But wait, some sources might say 2.5 or 4. Let me double-check. Oh right, the key is that they can consistently label their sex, which is usually around 3 years. So the correct answer would be Option C: 3 years.

Now, the wrong options. Let's say Option A is 2 years. At this age, they might not have consistent understanding. They can recognize body parts but not necessarily their sex. Option B could be 2.5 years. Maybe some children start to grasp it, but it's not consistent yet. Option D might be 4 years, which is too late. The question is about when they know their own sex, not when they understand gender roles or differences.

Clinical pearl: Gender identity becomes stable around 3 years. This is important for diagnosing gender dysphoria. So the key point is that by 3, the child can consistently state their sex, which is a high-yield fact for exams.

Wait, I need to make sure all parts are covered. The core concept is about gender identity development. The correct answer is 3 years old. The other options are either too early or too late. The clinical pearl is that stability occurs by 3, which is crucial. Let me structure this into the required sections now.

**Core Concept**
The question tests knowledge of **gender identity development** in children. By age 3, most children achieve **gender constancy**—understanding that their sex is stable over time and across situations. This involves recognizing anatomical differences and consistently identifying their own sex.

**Why the Correct Answer is Right**
By **3 years of age**, children develop **gender identity** and can consistently label their sex. This milestone is part of **Lawrence Kohlberg’s theory of gender development**, where children progress from recognizing physical traits (age 2–3) to internalizing gender as a fixed trait (age 3–4). The ability to identify one’s sex relies on **cognitive and social learning** processes.

**Why Each Wrong Option is Incorrect**
**Option A (2 years):** At this age, children may recognize body parts (e.g., penis, vagina) but **cannot consistently identify their own sex**.
**Option B (2.5 years):** While some children may begin to grasp gender concepts, most **lack consistent understanding** of their sex until age 3.
**Option D (4 years):** By age 4, gender identity is well-established, but the question asks for the **earliest age** of reliable self-identification.

**Clinical Pearl / High-Yield Fact**
Gender identity becomes stable by **3 years of age**. This

✓ Correct Answer: C. 36 months