Which of the following is an example of biological control?

Biological control refers to the use of living organisms to manage pest populations. In this context, the value of predatory insects feeding on aphids exemplifies this concept perfectly. When predatory insects, such as ladybugs or lacewings, consume aphids, they help to naturally regulate their population without the need for synthetic chemicals or other artificial interventions. This interaction takes advantage of the natural predator-prey relationship found in ecosystems, promoting a more sustainable and environmentally friendly approach to pest management.

The other options reflect different methods of pest control. While synthetic chemicals involve the application of man-made substances to eliminate pests, and chemical repellents deter pests through unpleasant scents or tastes, neither utilize living organisms to combat the problem. Trap crops can be a form of pest management, but they specifically rely on attracting pests to a particular area rather than enlisting the help of natural predators. Thus, the choice highlighting predatory insects represents a clear instance of biological control.