Step 1: Defining Basal Metabolic Rate (BMR):
Basal Metabolic Rate ($BMR$) represents the minimum amount of energy expression required by the body to sustain vital, involuntary physiological processes.
Step 2: Identifying Vital Core Functions:
These baseline processes include respiration, blood circulation, cellular homeostatic maintenance, and brain function when the individual is completely awake but at absolute rest.
Step 3: Evaluating Options:
Options (A), (B), and (C) involve physical exertion, exercise, or movement, which fall under Active Metabolic Rate ($AMR$). Only option (D) accurately defines BMR as energy expenditure at rest.