Answer;
-Stimulus
Environmental change that triggers a definite action on the part of the organism serves as a stimulus.
Step-by-step explanation;
-A stimulus (plural stimuli) is a detectable change in the internal or external environment.
-A stimulus can be internal or external. Sense organs, such as the ear, and sensory receptors, such as those in the skin, are sensitive to external stimuli such as sound and touch. For instance; A tropism is a response that an organism makes to a stimulus. An example of a common tropism in plants is phototropism. Plants grow towards light sources, and if the direction of light is changed, the plant will also change its direction of growth to accommodate for survival.