The answer is b.
Normally, neuronal cell membranes have negative voltages across them (meaning that the inside is more negative relative to the exterior). In the event of an action potential (when a neuron fires), the first thing that happens is that the membrane depolarizes, meaning that the charge of the inside relative to the outside begins to reverse. This happens when voltage-gated ion channels allow sodium ions (Na+, positively charged) to rush through the membrane and into the cell. Thus, the polarity of the membrane charge begins to reverse (inside more positive, outside less positive), which is what we call depolarization.
