Which of the following properties of light can be used to explain the…

Shadow formation is explained by two properties of light acting together: light travels in a straight line (rectilinear propagation), and opaque objects do not allow light to pass through them. When straight-line light rays strike an opaque object, the object blocks the rays completely, leaving a dark, sharply outlined region behind it — the shadow.

In this question, statement A states that light travels in a straight line, and statement D states that light does not pass through opaque objects. Rectilinear propagation (A) gives the shadow its straight-edged geometric shape, since each light ray is blocked exactly where it meets the object; opacity (D) ensures that no light reaches the region directly behind the object, so that region stays dark. Together, statements A and D are the two properties needed to fully explain how a shadow forms.

• The pairing of A and B combines straight-line travel with the speed of light. How fast light travels (about 3 × 108 m/s in vacuum) does not affect whether an opaque object blocks it — a shadow forms regardless of that speed.

• The pairing of B and C combines the speed of light with its composition of seven colours (seen when white light disperses through a prism, as in a rainbow). Neither the speed of light nor its colour composition explains why a dark region appears behind an opaque object.

• The pairing of C and D combines the seven-colour composition of light with opacity. Dispersion into colours explains phenomena such as rainbows, not the straight-edged blocking that produces a shadow's shape.

This can be checked by considering a transparent object placed in the same straight-line beam of light: because it does not stop the light (it lacks the property in statement D), it does not cast a dark shadow the way an opaque object does — confirming that straight-line travel together with opacity (statements A and D) are the properties that explain shadow formation.