Final answer:
A niche refers to the role of a species in its ecosystem, including how it interacts with the biotic and abiotic factors of the environment. A squirrel living in a tree is an example of a niche because it eats nuts and uses its sharp claws and climbing abilities to gather food.
Step-by-step explanation:
A niche refers to the role of a species in its ecosystem, including how it interacts with the biotic and abiotic factors of the environment. It can be defined by the food a species eats and how it obtains that food.
Based on this definition, option a, a squirrel living in a tree, is an example of a niche. The squirrel eats nuts and uses its sharp claws and ability to climb trees to gather food.
Options b, c, and d are not examples of niches. Option b simply describes a fish living in a pond, without any information about its specific role in the ecosystem. Option c mentions a lion hunting for zebra, which is more about the lion's behavior than its niche. Option d describes two birds flying in the air, which does not capture the concept of a niche.