The best answer is B.
Alternating current is a flow of charge that changes direction periodically. As a result, the voltage level also reverses along with the current.
Homes, offices and schools outlets are mostly AC. This is because generating and transporting AC across long distances is relatively easy. At high voltage (over 110kV), less energy is lost in electrical power transmission.
AC can be converted to and from high voltages easily using transformers. AC can be used to run electrical motors.