Which of the following stimuli is likely to elicit a nastic response in an organ...

This Biology question is asking which type of stimulus is most likely to cause a nastic response in an organism. A nastic response is a non-directional response to a stimulus, meaning that the organism does not move towards or away from the stimulus, but rather responds in a way that is not dependent on the direction of the stimulus.

Out of the options given, touch, light intensity, chemical substances, and gravity, the correct answer is chemical substances. This is because chemical substances can trigger a nastic response in an organism, such as the closing of a Venus flytrap in response to the presence of an insect.

Touch, on the other hand, is an example of a thigmotactic response, which is a directional response to touch. Light intensity is an example of a phototactic response, which is also a directional response to light. Gravity can cause gravitropic responses, which are directional responses to gravity.

It is important to understand different types of responses in organisms, as this can help us understand how they interact with their environment and how they adapt to different stimuli.