Explanation:

Omnivores:
Omnivores are animals that have the ability to consume both plant and animal matter. They have a flexible diet and can obtain nutrients from a variety of sources.

Options:

a) Cat: Cats are considered to be obligate carnivores, which means they primarily eat meat. While they may occasionally consume small amounts of plant matter, their diet is predominantly composed of animal protein.

b) Rat: Rats are omnivores. They have a diverse diet that includes grains, fruits, vegetables, and small animals. They are known to scavenge for food and adapt to a wide range of environments.

c) Mosquito: Mosquitoes are not omnivores. They are classified as hematophagous insects, which means they feed exclusively on the blood of other animals. They do not consume plant matter or other sources of food.

d) Crow: Crows are highly adaptable and opportunistic omnivores. They have a varied diet that includes fruits, seeds, insects, small animals, carrion, and even garbage. They are known for their intelligence and ability to find food in different environments.

Conclusion:
The correct answer is option C, Mosquito. Mosquitoes are not classified as omnivores because they do not consume both plant and animal matter. They solely feed on the blood of other animals, making them hematophagous insects.