A change that affects an organism's activity is called what?

The correct answer is stimulus.

A stimulus can be defined as a change in organism's atmosphere, which affect the behavior of an organism.

For an example: When a person goes out in summer, they produce more sweat. In summers the atmospheric temperature increases more than our body temperature. To prevent the temperature change in body sweat is secreted. In this case, the atmospheric temperature is a stimulus, to which our body responds by sweating.