The output of a class B amplifier for a sinusoidal input is a half-sinusoidal waveform.

Explanation:
A class B amplifier is a type of amplifier that is designed to amplify the positive half-cycle of the input waveform and the negative half-cycle separately. It uses a pair of transistors, one for each half-cycle, to achieve this.

Here's how a class B amplifier works:
1. The input sinusoidal waveform is split into positive and negative halves.
2. The positive half-cycle of the input waveform is amplified by one transistor, while the negative half-cycle is amplified by the other transistor.
3. Each transistor operates in the active region for only half of the input waveform cycle.
4. During the positive half-cycle, the positive transistor conducts and amplifies the input signal, resulting in a positive half-cycle output waveform.
5. During the negative half-cycle, the negative transistor conducts and amplifies the inverted input signal, resulting in a negative half-cycle output waveform.
6. The output from both transistors is combined to form the output waveform of the class B amplifier.

Since each transistor in a class B amplifier amplifies only one half-cycle of the input waveform, the output waveform will be a combination of the positive and negative half-cycles. This results in a half-sinusoidal waveform.

Advantages of class B amplifier:
- Higher efficiency compared to class A amplifier.
- Reduced power dissipation as each transistor operates only during its respective half-cycle.

Disadvantages of class B amplifier:
- Introduction of distortion due to the switching between transistors.
- The need for a biasing circuit to ensure that the transistors are properly biased.

In conclusion, the output of a class B amplifier for a sinusoidal input is a half-sinusoidal waveform.