Which fluid compartment contains the largest proportion of total body water?

- The largest proportion of total body water is contained within the Intracellular fluid (ICF) compartment.
- This is because the ICF represents the fluid inside the cells, which makes up about 60% to 65% of the total body water in an average adult.

In contrast, the Extracellular fluid (ECF), which includes plasma and interstitial fluid, accounts for about 35% to 40% of the body's total water content. The plasma is the fluid portion of the blood and makes up about 20-25% of the extracellular fluid, while the interstitial fluid surrounds the cells, making up the remaining portion of the ECF.

To summarize:
- Intracellular fluid (ICF) = ~60-65% of total body water
- Extracellular fluid (ECF) = ~35-40% of total body water
- Plasma = ~20-25% of ECF
- Interstitial fluid = remainder of ECF

Therefore, the correct answer is Intracellular fluid (ICF) because it contains the majority of the body’s water due to its presence inside all the cells.