If all cast shadows have unrelated darkness under one light source, what should be checked?

Correct answer: Surface, material, distance, and light relationship

The governing concept is consistent observation of cast-shadow values. One light source does not guarantee identical darkness because a shadow depends on the receiving surface, its material and colour, the distance from the object, and the angle and strength of the light. These factors should be checked together, so option D is correct. Object size alone, paper colour alone, or pencil sharpness cannot explain every difference in cast-shadow value.