Final answer:
When Cl- ions enter a neuron, they cause hyperpolarization by making the inside of the neuron more negatively charged.
Step-by-step explanation:
If Cl- ions were to rush into a neuron, its membrane potential would become more negative (undergo hyperpolarization). Chloride ions are negatively charged, and their influx would increase the negativity of the neuron's interior compared to the outside, leading to hyperpolarization, not depolarization or repolarization.