Answer:
D) pollination failure
Explanation:
Pollination failure can be caused by several factors which limit the chances of pollen being transferred to the stigmas, and/or prevents forming of fruits. In the nut trees, as a result of the self-incompatible flowers which limit the number of pollinators to only orchid bees, pollination failure is the most likely source of the problem why fruits don’t set. The selective nature of the nut flowers reduces the numbers of pollinators.