To solve this problem, we need to understand why it is easier for a small animal to run uphill compared to a large animal. Let's analyze each option:$\newline \bullet$Option 1: It is easier to carry a small body weight.$\newline$ - While smaller animals do have less body weight to carry, this is not the primary reason for their ease in running uphill.$\newline \bullet$Option 2: Smaller animals have a higher metabolic rate.$\newline$ - Smaller animals generally have a higher metabolic rate, which means they can produce energy more quickly.$\newline$ - This higher metabolic rate allows them to sustain intense physical activities like running uphill more efficiently.$\newline$ - Therefore, this option is correct.$\newline \bullet$Option 3: Small animals have a lower O$_2$ requirement.$\newline$ - Smaller animals do not necessarily have a lower oxygen requirement; in fact, their higher metabolic rate often demands more oxygen.$\newline \bullet$Option 4: The efficiency of muscles in large animals is less than in the small animals.$\newline$ - Muscle efficiency is not significantly different between small and large animals in a way that would affect uphill running.$\newline$Therefore, the correct answer is Option 2: Smaller animals have a higher metabolic rate.