To determine the decreasing order of operating speed (frequency) of semiconductor devices, we will analyze each device type based on their typical frequency characteristics:

1. MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor): MOSFETs are known for their high switching speeds due to their unipolar charge carrier conduction. They have very low gate capacitance, which allows them to turn on and off very quickly, making them suitable for high-frequency applications.
2. IGBT (Insulated Gate Bipolar Transistor): IGBTs combine the high input impedance and high switching speed of a MOSFET with the low on-state power loss and high current carrying capability of a BJT. Although they are faster than Power BJTs, they are generally not as fast as MOSFETs.
3. Power BJT (Bipolar Junction Transistor): Power BJTs operate at a lower frequency compared to MOSFETs and IGBTs due to their charge carrier recombination in the base region, which limits their speed.
4. SCR (Silicon Controlled Rectifier): SCRs are slower devices used for high power applications where high switching speeds are not essential. They are primarily used for controlled rectification and are among the slowest in terms of switching speed.

Based on the above analysis, the correct order of these semiconductor devices from highest to lowest operating speed (frequency) is:

B (MOSFET), A (IGBT), C (Power BJT), D (SCR)

Thus, the correct answer is B, A, C, D.