The correct answer is option D) competition.
The given case says the great hornbill and the toucan share the same food source in the same habitat, which means there is a competition for the food between great hornbill and toucan. As the hornbill's large bills help them in getting food, they have more chances of getting food as compared to toucans. As a result, the food source of the toucan would be exploited by great hornbills. So, in this case, the limiting factor for toucans would be competition for the food.
