The correct answer is: A. approximately equal to one.
An autotransformer is a type of transformer that uses only one winding to transfer power from one circuit to another. This makes it more efficient than a conventional transformer, which uses two windings.
The efficiency of an autotransformer is determined by its transformation ratio, which is the ratio of the output voltage to the input voltage. When the transformation ratio is approximately equal to one, the autotransformer is at its most efficient. This is because the current in the primary winding is equal to the current in the secondary winding.
When the transformation ratio is less than one, the autotransformer is less efficient. This is because the current in the primary winding is greater than the current in the secondary winding. This causes more copper losses in the primary winding.
When the transformation ratio is greater than one, the autotransformer is also less efficient. This is because the current in the secondary winding is greater than the current in the primary winding. This causes more copper losses in the secondary winding.
Therefore, the autotransformer makes effective saving on copper and copper losses, when its transformation ratio is approximately equal to one.