Explanation:

An autotransformer is a type of transformer that has a single winding and is used to step up or step down the voltage in a circuit. It is commonly used in applications where the voltage transformation ratio is not large, such as in sodium vapor lamps.

Leakage Reactance of Windings:
Leakage reactance refers to the reactance caused by the magnetic flux that does not link both the primary and secondary windings of a transformer. In an autotransformer, the leakage reactance of the winding is an important factor to consider when using it with a sodium vapor lamp. This is because sodium vapor lamps operate at high frequencies and require a stable voltage supply.

The leakage reactance of the windings in an autotransformer should be high in order to provide good voltage regulation and reduce the voltage fluctuations that can occur during the operation of the lamp. This is important because sodium vapor lamps are sensitive to voltage changes, and if the voltage supply is not stable, it can affect the performance and lifespan of the lamp.

By having a high leakage reactance, the autotransformer can effectively isolate the lamp from any voltage fluctuations or disturbances in the main power supply, ensuring a stable and reliable voltage output to the lamp.

Other options:
- Winding Resistance: The winding resistance of the autotransformer is not directly related to the operation of the sodium vapor lamp. It primarily affects the power losses in the transformer.
- VA Rating: The VA rating of the autotransformer should be sufficient to support the power requirements of the sodium vapor lamp. However, it is not the most important factor in ensuring a stable voltage output.
- Transformation Ratio: The transformation ratio of the autotransformer determines the voltage transformation capabilities. While it is important for the autotransformer to provide the desired voltage level for the lamp, it is not directly related to the stability of the voltage supply.