Final answer:
The electrical charge that travels down an axon is a variable charge due to the alternating influx of positive sodium ions and the exit of potassium ions, which causes the charge to temporarily switch from negative to positive as the action potential moves.
Step-by-step explanation:
The electrical charge that travels down the axon is best described as a variable charge. An axon is a part of a neuron that conducts electrical impulses away from the neuron's cell body. These impulses are known as action potentials and involve a temporary influx of positive charge into the cell due to the influx of positively charged sodium ions, followed by a restoration of the resting negative charge when potassium ions exit the cell. This switch back and forth between a positive and negative state as the action potential moves down the axon results in a variable charge.
Therefore, the correct answer to the question is d) Variable charge.