Assertion (A) : A cattle egret and grazing cattle in close association is a classic example of commensalism.

Reason (R) : As grazing cattle move through the field, they stir up and flush out insects from the vegetation that otherwise might be difficult for egrets to find and catch.

The correct option is: Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

Let us break down the concepts to understand why this option is correct.

1. Understanding Commensalism:
Commensalism is a type of symbiotic relationship between two organisms where one organism benefits from the association, while the other organism is neither helped nor harmed (remains unaffected).

2. Analyzing Assertion (A):
Assertion (A) states that the close association between a cattle egret (a bird) and grazing cattle is a classic example of commensalism.
In this ecological relationship, the cattle egrets forage close to where the cattle are grazing. The cattle, on the other hand, are completely unaffected by the presence of the egrets (they are neither harmed nor benefited). Because one species (egret) benefits and the other (cattle) is unaffected, this is indeed a classic example of commensalism. Therefore, Assertion (A) is true.

3. Analyzing Reason (R):
Reason (R) states that as the grazing cattle move through the field, they stir up and flush out insects from the vegetation that otherwise might be difficult for egrets to find and catch.
As cattle move through the grass, their physical movement disturbs the vegetation, forcing hidden insects to fly or jump out. The cattle egrets follow the cattle closely to easily catch and eat these flushed-out insects. Without the cattle's movement, the egrets would have to spend significantly more energy to find food. Thus, Reason (R) is true.

4. Evaluating the Explanation:
Since the physical action described in Reason (R) explains exactly how the egret benefits while the cattle remain unaffected, it directly provides the mechanism and reason for why this relationship is classified as commensalism. Therefore, Reason (R) is the correct explanation of Assertion (A).