What best defines the term "pollution"?

The term "pollution" is best defined as the introduction of harmful substances into the environment. This definition encompasses the various forms of pollution, including air, water, soil, and noise, each of which involves the contamination of natural resources in a way that can be harmful to ecosystems, wildlife, and human health.

Understanding pollution in this context highlights the critical nature of harmful substances, which can include chemicals, plastics, and waste products that disrupt the existing balance of ecosystems and can have long-lasting negative effects on the environment. This definition focuses specifically on the detrimental impact of these substances, which distinguishes pollution from other environmental concepts, such as conservation or recycling.

In contrast, other options relate to different aspects of environmental science. The balance of natural ecosystems does not specifically address harmful substances but rather the dynamic relationships within ecosystems. Recycling of waste materials is a process aimed at reducing pollution rather than defining it. The preservation of natural resources focuses on conservation efforts to protect those resources rather than the act of contaminating them. Thus, the definition highlighting the introduction of harmful substances is the most precise in describing what pollution entails.