Questions: The fluid inside of cells makes up approximately % of a person's total body water.

The answer is the first one: 66%.

Explanation for each option:

• 66%: This is the correct answer. The fluid inside cells, known as intracellular fluid, typically makes up about two-thirds (approximately 66%) of a person's total body water.
• 25%: This is incorrect. The extracellular fluid, which includes interstitial fluid and plasma, makes up about one-third of the total body water, but not the intracellular fluid.
• 33%: This is incorrect. As mentioned, the intracellular fluid makes up about 66% of the total body water, not 33%.
• 8%: This is incorrect. This percentage is too low to represent the intracellular fluid portion of total body water.

Summary: The fluid inside cells, or intracellular fluid, constitutes approximately 66% of a person's total body water.