Explanation:

Running uphill requires more muscular effort and energy expenditure compared to running on a flat surface or downhill. The question suggests that small animals find it easier to run uphill than large animals. The correct answer, option 'B', states that smaller animals have a higher metabolic rate, which is the key reason why it is easier for them to run uphill. Let's discuss this in detail.

Metabolic Rate:
- Metabolic rate refers to the rate at which an organism converts food into energy.
- Small animals generally have a higher metabolic rate compared to larger animals.
- Metabolic rate is influenced by several factors, including body mass and surface area.
- Small animals have a relatively larger surface area compared to their body mass, which results in a higher metabolic rate.
- This higher metabolic rate provides small animals with more energy for physical activities, such as running uphill.

Running Uphill:
- Running uphill requires more energy and effort due to the force of gravity working against the animal's motion.
- The animal needs to overcome the force of gravity to move vertically against it.
- This requires the contraction of muscles, which involves the expenditure of energy.
- The energy required for muscle contraction is obtained from metabolic processes.
- Since small animals have a higher metabolic rate, they can generate more energy to fuel their muscle contractions while running uphill.
- This enables them to overcome the gravitational force more efficiently and run uphill relatively easily.

Other Options:
- Option 'A': It states that it is easier to carry a small body weight. While this may be true in general, it does not directly explain why small animals find it easier to run uphill. Running uphill is primarily influenced by metabolic rate, not body weight alone.
- Option 'C': It states that small animals have a lower O2 requirement. While oxygen is necessary for energy production, a lower oxygen requirement does not necessarily make it easier for small animals to run uphill. Oxygen availability is important for sustained exercise, but the focus here is on the metabolic rate.
- Option 'D': It states that the efficiency of muscles in large animals is less than in small animals. While it is true that muscle efficiency can vary between different animals, it does not directly explain why small animals find it easier to run uphill. The key factor here is the higher metabolic rate in small animals.

In conclusion, the higher metabolic rate in small animals provides them with more energy for muscle contractions, enabling them to run uphill more easily compared to larger animals.

Basal metabolic rate is inversely proportional to body size. So smaller animals have a higher metabolic rate. Hence production of energy is more.