Indicate the main level of ecological study addressed by this question: How does the decline of bee pollinators affect fruit production in apple trees?

The main level is "population".

Explanation:

The main levels of study in ecology are the organism, the population, the community and the ecosystem.

In the case of the question above, we can say that although all levels are included, the research is mainly centered on the "population" level. This is because it seeks to define how a population of pollinating insects (and not just an organism) can interfere in the fruit production of a population of apple trees.