**Core Concept**
The question is testing the student's knowledge of the normal flora of the human skin, specifically the type of microorganism that coexists with the host without causing harm. The skin is a complex ecosystem with various microorganisms, including bacteria, fungi, and viruses, that inhabit its surface.

**Why the Correct Answer is Right**
Staphylococcus epidermidis is a common commensal organism found on the skin, particularly on the arms and legs. It is a gram-positive bacterium that thrives in the skin's natural flora, where it helps to maintain the skin's pH balance and prevents the overgrowth of other microorganisms. The skin's lipid-rich environment provides an ideal habitat for S. epidermidis, which adheres to the skin's surface through its lipoteichoic acid.

**Why Each Wrong Option is Incorrect**
* **Option A:** Staphylococcus aureus is a pathogenic bacterium that can cause skin infections, such as impetigo and folliculitis. It is not typically considered a commensal organism.
* **Option B:** Pseudomonas aeruginosa is a opportunistic pathogen that can cause infections in immunocompromised individuals, particularly in the lungs and urinary tract. It is not commonly found as a commensal on the skin.
* **Option D:** Candida albicans is a fungus that can cause opportunistic infections, particularly in immunocompromised individuals. While it can be found on the skin, it is not typically considered a commensal organism.

**Clinical Pearl / High-Yield Fact**
Staphylococcus epidermidis is a common cause of prosthetic joint infections, particularly in patients with joint replacements. This is because the bacteria can colonize the prosthetic material and cause a biofilm, leading to infection.

**Correct Answer: C. Staphylococcus epidermidis**