## Core Concept
Total body water (TBW) constitutes a significant percentage of body weight, and its proportion varies between individuals based on factors such as age, sex, and body composition. The average adult human body is primarily composed of water.

## Why the Correct Answer is Right
The correct answer, **60%**, reflects that approximately 60% of an adult's body weight is water. This percentage can fluctuate due to several factors, including age, sex, and body composition. For instance, athletes or individuals with more muscle mass may have a higher percentage of body water, while older adults or those with more body fat may have a lower percentage. Water is distributed throughout the body in various compartments, including intracellular fluid (ICF) and extracellular fluid (ECF), with about 2/3 of the body's water being intracellular and 1/3 extracellular.

## Why Each Wrong Option is Incorrect
- **Option A:** 20% - This is significantly lower than the actual percentage of total body water in adults. This option might be considered for individuals with severe dehydration but not for a typical adult.
- **Option B:** 40% - While closer, this still underestimates the actual percentage of total body water in a typical adult. It might be more reflective of individuals with higher body fat percentages.
- **Option D:** 80% - This overestimates the percentage of total body water. While newborns have a higher percentage of body water (around 70-80%), this is not accurate for adults.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the percentage of total body water decreases with age and increases with muscle mass. This concept is crucial in various clinical settings, such as calculating fluid replacement needs and understanding changes in body composition.

## Correct Answer: C. 60%