Introduction:
In electrical engineering, an oscillator is a circuit that generates a periodic waveform on its output without requiring an input signal. Sinusoidal oscillators produce a sinusoidal waveform as the output. However, there are also non-sinusoidal oscillators that produce waveforms other than sinusoidal. In this question, we are asked to identify which option is not an example of a non-sinusoidal oscillator.

Explanation:
Let's analyze each option to determine whether it represents a non-sinusoidal oscillator or not:

- Sawtooth Generators: A sawtooth generator is an example of a non-sinusoidal oscillator. It produces a waveform that starts from a low voltage and ramps up linearly to a high voltage before resetting to the starting voltage and repeating the process. The shape of the waveform resembles the teeth of a saw, hence the name "sawtooth."

- Blocking oscillators: Blocking oscillators are another example of non-sinusoidal oscillators. They typically consist of a capacitor, an inductor, and a semiconductor switching device such as a transistor. The circuit operates by alternately charging and discharging the capacitor, resulting in a non-sinusoidal waveform at the output.

- Multivibrators: Multivibrators are electronic circuits that generate square wave or rectangular wave outputs. They can be classified into various types such as astable, bistable, and monostable multivibrators. These circuits do not produce sinusoidal waveforms, making them non-sinusoidal oscillators.

- Crystal oscillators: Crystal oscillators are an exception in this list. They are a type of electronic oscillator that uses the mechanical resonance of a vibrating crystal of piezoelectric material to generate an electrical signal with a very precise frequency. Crystal oscillators are primarily used to generate highly stable sinusoidal waveforms, making them an example of a sinusoidal oscillator, not a non-sinusoidal oscillator.

Conclusion:
Based on the analysis above, the correct answer is option 'D', Crystal oscillators. They are not an example of a non-sinusoidal oscillator as they generate sinusoidal waveforms instead.