What is the best summary of this passage? Select one:
a. An external stimulus comes from outside the organism. Animals respond to stimuli through learned behaviors and instinct.
b. Phototropism is a plant’s response to turn toward light. Gravitropism is a plant’s response to grow upward against gravity.
c. Stimuli trigger responses in organisms that allow them to maintain constant internal conditions through homeostasis.
d. Homeostasis is a set of mechanisms that work to maintain stable internal conditions as external conditions change.