**Core Concept**
The question is testing the understanding of the composition of fecal mass, specifically the proportion of its components. Fecal mass is a complex mixture of water, electrolytes, nutrients, and waste products that are eliminated from the body through the gastrointestinal tract.

**Why the Correct Answer is Right**
The correct answer is **C. Sloughed mucosal cells and bacteria**. Fecal mass is primarily composed of sloughed mucosal cells from the gastrointestinal tract and a large number of bacteria, including both aerobic and anaerobic species. The bacteria play a crucial role in the breakdown and fermentation of undigested carbohydrates, producing short-chain fatty acids and contributing to the bulk of the fecal mass.

**Why Each Wrong Option is Incorrect**
**Option A:** Water is a significant component of fecal mass, but it is not the main contributor.
**Option B:** Nutrients, such as carbohydrates, proteins, and fats, are absorbed in the small intestine and are not a major component of fecal mass.
**Option D:** Dead epithelial cells from the small intestine are indeed a part of fecal mass, but they are not the primary component.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the gastrointestinal tract is home to a vast population of microorganisms, known as the gut microbiome, which plays a critical role in digestion, absorption, and elimination of waste products.

**Correct Answer:** C. Sloughed mucosal cells and bacteria