What type of transformer uses only a secondary coil?

The correct choice is a current transformer, which is designed to measure the current flowing through a conductor by producing a reduced current in its secondary winding that is proportional to the current in the primary winding. This allows for safe and accurate measurement and monitoring of electrical currents in high-voltage systems.

Current transformers are specifically designed to operate in a measurement context and are integral to protective relaying systems, making them distinct from step-up or step-down transformers, which alter voltage levels. Step-up and step-down transformers both require a primary coil and operate on the principle of electromagnetic induction to change voltage levels, while an isolation transformer is used primarily for safety and reducing noise without altering the power level significantly; it also has both primary and secondary coils. Therefore, the key feature that makes current transformers unique is that they utilize only the secondary coil for their operational intent, focusing solely on current measurement rather than voltage transformation.