Explanation:

- Apportioning the cost of electric power among different departments should reflect both the capacity and the usage of electrical equipment.

- Option 1: Horsepower of motors - Only indicates the power capacity but not actual consumption.

- Option 2: Number of light points - Relates to lighting, not comprehensive power usage.

- Option 3: Horsepower multiplied by machine hours - Correct. This option considers both the capacity (horsepower) and the duration of use (machine hours), providing a balanced metric for apportioning power costs accurately.

- Option 4: Machine hours - Reflects usage time but lacks power capacity information.