What type of transformer is used to produce an output of 12 volts from a 120 x 240-volt primary?

The type of transformer used to produce an output of 12 volts from a 120 x 240-volt primary is a step-down transformer. This transformer is specifically designed to reduce voltage from a higher level to a lower level, which in this case is from 120 or 240 volts down to 12 volts.

A step-down transformer has a primary winding that has more turns of wire than the secondary winding. This design allows it to effectively decrease the voltage while increasing the current. Therefore, in applications where devices require a lower voltage (like many low-voltage devices), a step-down transformer is essential.

An autotransformer and an insulating transformer serve different purposes. An autotransformer can change voltage levels but does so through a single winding, which may not provide electrical isolation between input and output. An insulating transformer, while it serves to provide electrical isolation, is not typically used to change voltage levels in the same way that step-down or step-up transformers do.

Overall, the step-down transformer is the correct choice because it matches the requirement of lowering the primary voltage to the desired output voltage.