The correct option is C straight line
If the position of object and screen is fixed, the shape of shadow depends on the angle at which the light from the source falls on the object. If light is projected from a certain angle, square sheet can cast a rectangular shadow.
If the position of source and screen is fixed, the shape of shadow depends the alignment of the object. A square sheet can be placed in such a way that it can cast a shadow in the sahpe of a staright line.