What steps up or steps down the input line voltage and isolates the power supply from the power line?

The transformer is the correct choice because it is specifically designed to change the voltage level of an input line. It can either step up (increase) or step down (decrease) the input voltage depending on the turns ratio of its primary and secondary coils. This functionality is vital in power supply circuits, where different components require different voltage levels.

Additionally, the transformer provides electrical isolation between the power supply and the power line. This isolation is essential for protecting sensitive electronic components from high voltage surges and helps improve the safety of the system.

The other options do serve important functions within an electronic circuit but do not fulfill the dual roles of stepping voltage and providing isolation in the same way that a transformer does. Rectifiers convert AC to DC, filters smooth out voltage levels, and regulators maintain a constant output voltage, but they do not have the ability to change voltage levels while offering isolation from the power supply.