The correct answer is Transformer and rectifier.

A voltage sensing relay needs a proportional DC voltage to operate correctly. This means the output DC voltage should be directly proportional to the input AC voltage being sensed. Let's examine why the other options are incorrect and then detail how a transformer and rectifier achieve this:

• Capacitor and diode: A capacitor and diode can form a simple rectifier circuit, but without a voltage divider or other scaling mechanism, the output DC voltage will not be reliably proportional to the input AC voltage. The output will depend heavily on the capacitor's value and the input AC voltage's magnitude.
• Resistor and inductor: A resistor and inductor combination can create a filter, but this is typically used to smooth signals, not necessarily to create a proportional DC voltage output from an AC input. The output voltage won't have a linear relationship with the input voltage.
• Motor and generator: This option is completely unsuitable. Motors convert electrical energy into mechanical energy and generators convert mechanical energy into electrical energy. They do not inherently create a proportional DC voltage from an AC voltage input.
• Transformer and rectifier: This is the correct choice. A transformer is used to step down (or up, depending on the application) the AC voltage to a safer and more suitable level. Then, a rectifier circuit converts the AC voltage to DC voltage. This combination ensures that the resulting DC voltage is proportional to the original AC voltage (with the proportionality constant being determined by the transformer's turns ratio). The rectifier's filtering action (which could incorporate a capacitor, for instance) then smooths the output into a relatively stable DC voltage.

Therefore, a transformer provides the voltage scaling and a rectifier converts the AC to DC; in combination, these elements create the desired proportional DC voltage required for the voltage sensing relay to accurately detect and respond to input voltage levels.