Why Photoconductive materials are semiconductors whose valence electrons can be knocked into the conduction band by incoming photons. More light means more free electron-hole pairs, which means more charge carriers available to carry current, so conductivity rises and resistance falls. That is exactly how a CdS photocell (light-dependent resistor) in a streetlight controller works: dark means high resistance, bright means low resistance.
Watch out The choice saying resistance increases has it backwards, and reflectivity is a surface optical property, not what photoconductivity describes.
Photo-CONDUCTIVE: light raises conductivity, so resistance goes down. More light, less ohms.