The most relevant benefit that plant species would gain from a pollinator that only visits the flowers of that plant species is:
D decreased self-fertilization allowing for greater genetic variation.
When a pollinator is specialized to visit only one species, it can enhance cross-pollination, which promotes genetic diversity. This genetic variation is beneficial for the adaptability and resilience of the plant population. The other options may have some relevance but do not directly capture the primary ecological advantage associated with specific pollinators and plant interactions.