**Core Concept**
The human skin is a complex ecosystem that hosts a diverse range of microorganisms, including bacteria. The skin microbiome plays a crucial role in maintaining skin health and preventing infection.

**Why the Correct Answer is Right**
The correct answer is based on the average number of bacteria that colonize the human skin. This number can vary depending on factors such as skin location, hygiene, and environmental conditions. The skin's bacterial flora is essential for its function and overall health.

**Why Each Wrong Option is Incorrect**
**Option A:** This option underestimates the number of bacteria on the skin.
**Option B:** This option also underestimates the number of bacteria on the skin.
**Option D:** This option overestimates the number of bacteria on the skin.

**Clinical Pearl / High-Yield Fact**
The skin microbiome is composed of both commensal and pathogenic bacteria. Understanding the balance between these two types is crucial for managing skin infections and diseases.

**Correct Answer:** C. 1 trillion