A step-down transformer _____________ output voltage in the secondary.

A step-down transformer is specifically designed to reduce the voltage from its primary winding to its secondary winding. This means that the output voltage in the secondary is lower than the input voltage in the primary. When AC voltage is applied to the primary winding, the alternating magnetic field induces a lower voltage in the secondary winding based on the turns ratio between the primary and secondary coils. A step-down transformer achieves this by having fewer turns in the secondary winding compared to the primary winding. Therefore, the output voltage in the secondary is indeed decreased, making the choice of "decreases" the correct answer.

In contrast, a step-up transformer would increase the voltage by having more turns in the secondary. The other options—maintaining, increasing, or modulating voltage—do not accurately describe the function of a step-down transformer.