In a Class A amplifier, what percentage of the time does the amplifying device conduct?
Why Amplifier classes are defined by conduction angle, the fraction of the input cycle during which the active device draws current. Class A biases the device in the middle of its linear region so it never cuts off, conducting for the full 360 degrees, which is 100% of the cycle. That gives the best linearity but the worst efficiency (theoretically 50% or less), since the device dissipates power even with no signal.
Watch out The choice saying more than 50% but less than 100% describes Class AB (conduction angle between 180 and 360 degrees), 50% is Class B (180 degrees), and less than 50% is Class C.
A = All the time (360 deg), AB = in between, B = half (180 deg), C = less than half.