The correct answer is mouse.
The metabolic rate of an animal is the rate at which it consumes energy. It is measured in units of oxygen consumption per gram of body weight per hour (mm3/g hour). The metabolic rate of an animal is determined by its size, body temperature, and activity level.
Small animals have a higher metabolic rate than large animals. This is because small animals have a larger surface area to volume ratio, which means that they lose heat more easily. To compensate for this, small animals have to eat more food and produce more heat.
Warm-blooded animals have a higher metabolic rate than cold-blooded animals. This is because warm-blooded animals have to maintain a constant body temperature, which requires energy. Cold-blooded animals, on the other hand, can allow their body temperature to fluctuate with the environment, which requires less energy.
Active animals have a higher metabolic rate than inactive animals. This is because active animals need to use more energy to move around. Inactive animals, on the other hand, can conserve energy by staying still.
Of the four mammals listed, the mouse has the highest metabolic rate. This is because mice are small, warm-blooded, and active animals.