Which factor contributes to the breeding of fruit flies in environments?

The breeding of fruit flies is significantly influenced by the presence of food residue left on surfaces. Fruit flies are attracted to decaying or fermenting organic matter, which includes fruits, vegetables, and any food waste that may be present in an environment. This residue provides both a food source and a suitable breeding ground for the flies, as they lay their eggs in the vicinity of food that will be accessible for their larvae once they hatch.

In contrast, options like too much cleaning would typically reduce the debris that attracts fruit flies, while proper storage of food would prevent exposure to items that could be breeding sites. Additionally, a lack of humidity is not generally a factor that would encourage the breeding of fruit flies, as these pests thrive in moist environments. Therefore, the presence of food residue is crucial for creating conditions that favor the proliferation of fruit flies.