Final answer:
The measurement to determine the basal metabolic rate (BMR) in each rat is A. Oxygen consumption. This is due to the direct relationship between energy consumption and oxygen consumed during metabolic processes.
Step-by-step explanation:
To determine the basal metabolic rate (BMR) in each rat, you would measure A. Oxygen consumption. Energy consumption is directly proportional to oxygen consumption as the process of metabolism essentially involves the oxidation of food.
The experiment performed using a respirometer assesses the rate of respiration by measuring the volume of O₂ consumed per gram of the organism. This is because approximately 20 kJ of energy is produced for each liter of oxygen consumed.
BMR can be influenced by several factors including age, gender, body weight, climate, hormone levels, and more. However, in the context of the information provided, oxygen consumption is the best measure for determining BMR.