Alan places a light bulb in front of three similar cardboard pieces such that each have a hole as illustraighted in the image below. 

He can't see the candle from the other side of the cardboard C because of:

• The bulb cannot be seen from the other side of cardboard C because the light travels in a straight line. Which is also called rectilinear propagation of light.  
• Since the holes made in the cardboards are not in a straight line, the light will not be able to pass through all the cardboard.

Therefore, 'Rectilinear propagation of light' is the correct response.