Which nutrient is considered the most vital for an animal's body, comprising about 75% of it?

Water is indeed the most vital nutrient for an animal's body, as it makes up about 75% of an animal's overall composition. It plays a critical role in numerous physiological processes, including temperature regulation, cellular function, digestion, and waste elimination. Adequate hydration is essential for maintaining homeostasis and supporting metabolic reactions. Unlike other nutrients, water is unique in its universal importance across all life forms; without it, survival can only be sustained for a limited time.

In contrast, the other nutrients, such as starch, fat, and protein, while important for energy, structural, and functional roles within the body, do not match water’s fundamental necessity for survival and immediate physiological functioning. Starch serves as a carbohydrate source for energy, fat provides energy storage and insulation, and proteins are essential for growth and repair of tissues, but none can substitute for the critical and diverse roles that water fulfills in maintaining an animal's health and well-being.