**Core Concept**
Total body water (TBW) is the amount of water present in the human body, including intracellular and extracellular fluid. It is a critical component of human physiology, playing a vital role in maintaining proper bodily functions, such as temperature regulation, nutrient transport, and waste removal.

**Why the Correct Answer is Right**
The correct answer is 60% of body weight, which is the average TBW in adults. This is because the human body is composed of approximately 55-60% water, with the majority being intracellular fluid (about 40%) and a smaller proportion being extracellular fluid (about 20%). The TBW varies slightly among individuals, depending on factors such as age, sex, and body composition.

**Why Each Wrong Option is Incorrect**
**Option A:** 40% of body weight is incorrect because it underestimates the actual TBW in adults. While this value may be closer to the TBW in infants, it is not representative of the average adult population.
**Option B:** 20% of body weight is incorrect because it significantly underestimates the actual TBW in adults. This value is more representative of the extracellular fluid compartment, not the total body water.
**Option C:** 10% of body weight is incorrect because it is a gross underestimation of the actual TBW in adults. This value is not even close to the average TBW in adults.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that TBW varies throughout life, with infants having a higher percentage of TBW (about 70-80%) compared to adults. This is because infants have a higher proportion of intracellular fluid due to their immature kidney function and higher metabolic rate.

**✓ Correct Answer: D. 60% of body weight**