**Core Concept**
The development of the external genitalia in females involves the differentiation of the genital swellings, which are precursors to the labia majora. This process is a crucial aspect of embryonic development and is influenced by the presence or absence of specific sex hormones.

**Why the Correct Answer is Right**
The genital swellings, also known as the labioscrotal swellings, are embryonic structures that give rise to the labia majora in females. In the absence of androgens, these swellings develop into the labia majora. This is in contrast to males, where the genital swellings fuse to form the scrotum. The development of the genital swellings is a key aspect of female external genitalia formation and is influenced by the expression of specific genes and hormones.

**Why Each Wrong Option is Incorrect**
**Option A:** The urogenital sinus is a different embryonic structure that gives rise to the urethra and the lower part of the vagina. It is not directly involved in the development of the labia majora.
**Option B:** The Mullerian ducts are embryonic structures that give rise to the female internal genitalia, including the fallopian tubes, uterus, and upper part of the vagina. They are not involved in the development of the external genitalia.
**Option C:** The genital ridge is an embryonic structure that gives rise to the gonads (ovaries or testes). While it plays a crucial role in the development of the reproductive system, it is not directly involved in the development of the labia majora.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the development of the external genitalia is influenced by the presence or absence of sex hormones, specifically androgens. In the absence of androgens, the genital swellings develop into the labia majora, whereas in their presence, they fuse to form the scrotum in males.

**✓ Correct Answer: D. Genital swelling. Genital swelling.**