Which issue is likely to cause a decrease in DC voltage in a rectifier?

A blown AC fuse is indeed likely to cause a decrease in DC voltage in a rectifier. In a rectifier circuit, the AC voltage is transformed and then rectified to produce DC voltage. If the AC input to the rectifier is disrupted due to a blown fuse, the transformer will not receive the necessary power to function correctly, and consequently, no or very little voltage will be supplied to the rectifier. This lack of input voltage will result in a significantly decreased output voltage on the DC side.

In contrast, while burnt resistors, incorrect diode connections, or an overheated transformer might lead to issues in the circuit, they will manifest in different ways, such as limiting current flow or causing the circuit to malfunction. A burnt resistor may cause a voltage drop, but it would not entirely stop the voltage from reaching the rectifier. Incorrect diode connections could lead to failure in rectifying the waveform correctly, resulting in unexpected output characteristics rather than a straightforward decrease in voltage. An overheated transformer might affect performance, but if it still operates, there may be some output voltage present. Therefore, the blown AC fuse stands out as the most direct reason for a decrease in DC voltage in this scenario.