What is the primary function of an insecticide?

Insecticides are specifically designed to target and manage insect populations that can adversely affect plants, crops, or even human health. The primary function of an insecticide is to kill, harm, repel, or otherwise mitigate the negative effects of insects. This is achieved through various chemical mechanisms, which can disrupt the insect's nervous system, mimic hormones, or interfere with basic physiological functions.

While some options might mention effects related to plant growth or soil quality, these are not the primary role of insecticides. Enhancing plant growth and improving soil quality typically involve fertilizers and soil amendments rather than insecticides. Similarly, enhancing pest immunity is more about breeding or genetic modification and does not pertain to the action or purpose of insecticides, which focus on controlling pest populations rather than enhancing their resistance.

Thus, the most accurate answer reflects the intended use of insecticides in pest management by stating their primary purpose of killing, harming, repelling, or mitigating insects.