A mobile phone charger is

The correct answer is D) A step-down transformer. A mobile charger takes mains supply of about 220-230 V AC and delivers about 5 V DC to the phone. Its core job is to reduce voltage, and the transformer does that by turning ratio: Vs/Vp = Ns/Np, so fewer secondary turns give a lower output voltage. The charger then rectifies and regulates the reduced AC to DC.

• A) Inverter: wrong. An inverter converts DC into AC. A charger does the reverse, converting AC into DC.
• B) UPS: wrong. A UPS is a backup system with a battery, a charger circuit and an inverter. It keeps load running during power cuts. A phone charger has no battery and no backup function, so it is not a UPS.
• C) Step-up transformer: wrong. It raises voltage, which is the opposite of what a charger needs.

Concept: transformer action, rectification and the AC-DC conversion chain. Modern chargers use a switched-mode power supply with a high-frequency transformer, which is smaller and lighter. The step-down function is still the same, so the answer does not change. This links to Mains topics on energy efficiency and standby losses in household electronics.

Trap: aspirants pick UPS or inverter because they think of batteries and AC power. Remember the direction of conversion: a charger goes from AC to DC and steps the voltage down.