Final answer:
The vital ability of an organism to react to changes in its environment is called responsiveness. It involves adjustments to both internal and external stimuli which may result in movement or other actions.
Step-by-step explanation:
The vital ability of an organism to take certain actions when changes occur in its immediate environment is called responsiveness. Responsiveness is the ability of an organism to adjust to alterations in both its internal and external environments. An example of this can include movement toward sources of food and water, or away from perceived threats. Internal changes, like a spike in body temperature, can provoke actions such as sweating and the dilation of blood vessels to help cool the body down. All living things detect changes in their environment and respond accordingly, which may sometimes be evidenced by physical motion, such as plants exhibiting phototropism by turning their leaves toward the sun.