Which of the following best exemplifies natural control in agriculture?

Natural control in agriculture refers to methods that utilize the natural ecosystem and biological processes to manage pests and promote crop health without heavily relying on synthetic chemicals or artificially enhancing plant growth.

The use of biological pest control agents embodies this concept as it involves employing natural predators or parasites to keep pest populations in check. For example, introducing ladybugs to control aphid populations effectively leverages the natural food web and ecological interactions, thus maintaining a healthier balance within the agricultural system. This method not only reduces the reliance on synthetic pesticides but also promotes biodiversity and enhances soil health.

In contrast, the other options involve either artificial substances or mechanical processes that do not utilize the crops' or ecosystem's inherent capacities for pest management. Chemical pesticides disrupt natural controls and can have detrimental environmental effects, while mechanical weeding, although helpful, does not harness biological interactions. Increased fertilizer application focuses on plant growth enhancement rather than pest control, making it less relevant to the concept of natural pest management.