Which of the following properties of light can be used to explain the phenomenon of shadow formation? A. Light travels in a straight line. B. Light travels at a high speed. C. Light is composed of seven colours. D. Light does not pass through opaque objects.

Shadows are formed because light travels in straight lines (rectilinear propagation), and when an opaque object blocks the light, no light reaches the region behind it, forming a shadow. Hence both statements A and D are needed to explain shadow formation. The speed of light and its composition into seven colours are unrelated to shadow formation.