Final answer:
The basal metabolic rate per unit of body mass is highest in large endotherms because they have a smaller surface area and therefore lose heat at a slower rate than smaller endotherms.
Step-by-step explanation:
The basal metabolic rate per unit of body mass is highest in A) large endotherm. Smaller endothermic animals have a greater surface area for their mass than larger ones.
Therefore, smaller animals lose heat at a faster rate than larger animals and require more energy to maintain a constant internal temperature. This results in a smaller endothermic animal having a higher BMR, per body weight, than a larger endothermic animal.