Concept:Neurones follow the all-or-none principle; a minimum stimulus intensity is required to trigger a response.
Explanation:Neurones do not respond to stimuli of any strength.
Each neurone has a minimum level of stimulation called the threshold.
The stimulus intensity must reach this threshold value before the neurone can be excited and generate an impulse.
Below the threshold, no response is produced.
During the absolute refractory period, a neurone cannot respond at all, even to a strong stimulus.
Stimuli applied in rapid succession also do not always produce separate responses.
Hence, the key requirement for excitation is that the stimulus intensity reaches the threshold value.
Answer:D. Intensity of stimulus must reach a threshold value before the neurons can be excited.