Which ion enters an axon to start a nerve impulse?

The correct answer is Sodium (Na+) ions.

At rest, the inside of an axon is negative compared to the outside. This difference in electrical potential is called the resting potential. When an action potential is triggered, sodium channels in the axon membrane open, allowing sodium ions to rush into the axon. This influx of positive ions depolarizes the membrane, causing an action potential to propagate down the axon.