Final answer:
The energy expenditure from physical activity can account for 15-45% of the total energy expenditure in the energy balance equation.
Step-by-step explanation:
Energy expenditure from physical activity constitutes a highly variable component of the energy balance equation. According to the information provided, energy expenditure due to basal metabolic rate (BMR) accounts for about 75% of the calories burned in a day, with physical activity responsible for approximately another 20%.
Therefore, the correct answer to the question 'Physical activity constitutes the most variable part of energy expenditure side of the energy balance equation. In fact, energy expenditure from physical activity can account for _______ of the total energy expenditure.' is a. 15-45%.