Assertion: Herbivores are also called as first order consumers.
Reason: Herbivores obtain their food directly from plants.
A) Both Assertion and Reason are correct and the Reason is the correct explanation for Assertion.
B) Both Assertion and Reason are correct but the Reason is not the correct explanation for Assertion.
C) Assertion is correct but Reason is incorrect.
D) Both Assertion and Reason are incorrect.

Hint:Consumers are those animals that depend on other organisms to obtain their food which is a source of nutrition required for their survival.

Complete answer:
Primary consumers are those animals that directly depend on plants to obtain their food.
>Option A: Herbivores are those animals that depend on plant materials for their food and cannot synthesize their own food. They obtain food from these plants directly, hence they are also called first order consumers. Thus, Both the Assertion and Reason are correct and the Reason is the correct explanation for Assertion. Therefore, this is the correct option.
>Option B, C and D: Herbivores are those animals that depend on plant materials for their food and cannot synthesize their own food. They obtain food from these plants directly, hence they are also called first order consumers. Thus, both the Assertion and Reason are correct and Reason is the correct explanation of the Assertion. Therefore, this is the incorrect option.

Thus, the correct option is (A) Both Assertion and Reason are correct and the Reason is the correct explanation for Assertion.

Note:It is to be noted that herbivores are those animals that depend on plant materials as their source of food or nutrition. The other categories of animals are,
(i) Carnivores - They depend on meat mainly for food.
(ii) Omnivores - They depend on both meat and grass for food.
(iii) Decomposers - They depend on dead and decaying matter.