Introduction:
An omnivore is an animal that has the ability to consume and derive nutrients from both plant-based and animal-based sources. This dietary flexibility allows omnivores to adapt to various environments and increase their chances of survival. Among the animals mentioned, the correct answer is the rat.

Explanation:
Let's examine each option to determine if the animal is an omnivore or not:

a) Rat:
Rats are indeed omnivores. They have a varied diet that includes both plants and small animals. Rats can consume grains, fruits, vegetables, insects, and even small vertebrates. This dietary flexibility allows them to thrive in different habitats, including urban areas where they scavenge for food.

b) Cow:
Cows, on the other hand, are herbivores. They have a specialized digestive system that allows them to efficiently extract nutrients from plant material, such as grass and hay. Their stomachs contain multiple chambers for fermentation, which aids in breaking down the cellulose found in plants.

c) Lion:
Lions are carnivores, meaning their diet consists primarily of meat. They are at the top of the food chain and feed on other animals, such as antelope, zebras, and buffalo. Lions have sharp teeth and strong jaws designed for tearing flesh, which indicates their specialization as carnivores.

d) All of these:
While rats are omnivores, cows and lions have specific dietary preferences. Therefore, the correct answer is not "All of these" but rather option "A" - Rat.

Conclusion:
In conclusion, among the animals mentioned, the only omnivore is the rat. Rats have the ability to consume both plant-based and animal-based food sources, allowing them to adapt to a wide range of environments.