An undesirable change in the environment is called:

The correct answer is Pollution. Pollution is defined as the introduction of contaminants into the natural environment that cause adverse changes. These undesirable changes can harm living organisms, damage ecosystems, and degrade the quality of air, water, and land, making it a significant environmental concern.

• Conservation refers to the protection, preservation, management, or restoration of natural resources and ecosystems, which is a desirable environmental action.
• Erosion is the natural process by which soil and rock are worn away by natural forces like wind or water. While it can be exacerbated by human activity, it is a geological process, not the definition of an undesirable change itself.
• Reforestation is the process of replanting trees on land that has been deforested, which is a beneficial and desirable environmental practice.