The correct option is D Association of cattle egret and grazing cattle.
Mutualism is the positive interaction in which two organisms live close together and benefit each other equally such as mycorrhiza, plant and animal relation for pollination and lichens. Whereas, cattle egret and grazing cattle possess the symbiotic relationship but not mutualistic because they show commensalism in which cattle egret feed on the bugs and flies that irritate the grazing cattle. In such relation, one organism gets the benefit and other is unaffected. Thus, the correct answer is option D.