The correct answer is: D. quantity of active material
The capacity of a lead-acid cell is the amount of charge it can store. It is determined by the amount of active material in the cell, which is the lead plates and the sulfuric acid electrolyte. The more active material there is, the more charge the cell can store.
The temperature of the cell can affect its capacity, but only to a small extent. A higher temperature will increase the capacity of the cell, while a lower temperature will decrease it. However, the effect of temperature is not significant enough to be a major factor in determining the capacity of a lead-acid cell.
The rate of charge and discharge can also affect the capacity of a lead-acid cell, but again, only to a small extent. A higher rate of charge or discharge will decrease the capacity of the cell, while a lower rate will increase it. However, the effect of the rate of charge or discharge is not significant enough to be a major factor in determining the capacity of a lead-acid cell.
Therefore, the only factor that significantly affects the capacity of a lead-acid cell is the quantity of active material.