An inhibitory stimulus has occurred. B. Chemically gated potassium channels have opened. C. Excessive potassium has diffused out causing hyperpolarization. D. Excessive depolarization of the axon has occurred.

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C. Excessive potassium has diffused out causing hyperpolarization.

Explanation:

The nerve action potential can be divided into following stages:

Stimulus is detected by the cell in resting stage.Sodium channels in the membrane open from where influx of sodium ions occur which is called depolarizationAfter a while, sodium channels close and potassium channels open from where efflux of potassium ions occur which is called repolarization.The membrane potential further lowers due to continous efflux of potassium ions which is called hyperpolarization.After a while potassium channels close and membrane returns to its resting stage.

In the given figure, stage 4 depicts hyperpolarization because the membrane potential has dropped to the lowest point below -70mV. Hence, option C is correct.

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