**Core Concept**
Squamous cell carcinoma (SCC) is a type of skin cancer characterized by the uncontrolled growth of abnormal squamous cells. It commonly arises in areas of the body exposed to UV radiation, which damages the DNA of skin cells, leading to malignant transformation.

**Why the Correct Answer is Right**
The most common site of squamous cell carcinoma is the skin, specifically the areas that are frequently exposed to UV radiation from the sun or tanning beds. The face, particularly the nose, ears, and lips, is a common site due to its high exposure to UV rays. SCC can also occur on other sun-exposed areas such as the hands, arms, and legs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as SCC can occur in other sites, but it is not the most common site.
**Option B:** This option is incorrect as basal cell carcinoma is a different type of skin cancer that is more common, but also more benign.
**Option C:** This option is incorrect as SCC can occur in the mucous membranes and other areas, but it is not the most common site.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that SCC can be prevented by protecting the skin from UV radiation through the use of sunscreen, clothing, and hats, especially in individuals with fair skin or a history of sunburn.

**Correct Answer:** D.