What type of transformer is needed to increase voltage to kilovolt levels in the x-ray circuit?

To increase voltage to kilovolt levels in the x-ray circuit, a step-up transformer is utilized. This type of transformer is specifically designed to increase the input voltage to a higher output voltage, which is essential for generating the high voltages required for x-ray production.

In an x-ray circuit, the high voltage is needed to accelerate electrons from the cathode to the anode within the x-ray tube, enabling sufficient energy for x-ray production. The step-up transformer achieves this by having more turns of wire in the secondary coil than in the primary coil, effectively increasing the voltage during the electromagnetic induction process.

The other options do not serve this purpose. A step-down transformer reduces voltage, which is not suitable for achieving the kilovolt levels necessary in x-ray production. Buck transformers and series transformers are not standard terms related to increasing voltage in x-ray applications and their functions differ from what is needed in this context. Thus, the step-up transformer is the correct choice.