What happens when light falls upon a light dependent resistor placed in a circuit?
A) Current going through LDR increases.
B) LDR begins to glow.
C) The voltage across LDR decreases.
D) Its resistance decreases.

Hint: For solving this problem we will apply the principle of photoconductivity, movement of electrons from valence band to conduction band when light falls upon a light dependent resistor.
We will also use the concept that when conductivity of a material or substance increases its resistance decreases. Using the above two principles we will complete our solution.

Complete step by step answer:
Let’s define photosensitive resistor or light dependent resistor (LDR) or cadmium sulphide cell first.
Light dependent Resistor: Photosensitive resistor becomes sensitive when light falls on it. It is an electronic component whose resistance gets changed when light falls on it, as resistivity of the light dependent resistor is a function of light.
Light dependent resistors are made up of semiconductor materials because semiconductors have very few free electrons. We know that when free electrons move, current flows. In semi conductors free electrons are very less in number therefore no current flows. When light falls on a photoresistor, photons are emitted when imparting energy to the bounded electrons, this imparted energy allows electrons from valence to move to the conduction band and hence the current flows because resistance of the semiconductor decreases and the conductivity increases.
Thus we can say that when light falls upon LDR, resistance decreases and conductivity of the LDR increases.
Option D is correct.

Note: Applications of photoresistors are: used in designing of electronic components where sensitivity of light is required such as Automatic street light circuits, fire alarms, light activated switch circuits, automatic LED emergency light, night security light. Used in various light varying sensors also.