Explanation:

- Transformer (Option 3)

- A transformer is specifically used to change voltage levels while maintaining the same frequency. It operates on the principles of electromagnetic induction.

-

- Transistor (Option 1)

- Used to amplify or switch electronic signals. Not used for changing voltage levels while keeping frequency constant.

- Resistor (Option 2)

- Utilized to limit or regulate the flow of electrical current. Does not change voltage without affecting frequency.

- Galvanometer (Option 4)

- Instrument for detecting and measuring electric current. Not related to altering voltage or frequency.

- Capacitor (Option 5)

- Stores electrical energy temporarily in an electric field. Not meant for changing voltage with fixed frequency.