Explanation

The phenomenon described is called Tyndall effect, which shows that light travels in straight lines. This property relates to the nature of light's path, making 'Transparent' a better fit for describing how light passes through the medium. Option A is incorrect because visibility refers to being seen, not the physical property causing the beam. Option C is incorrect because while sunlight itself might be considered invisible until it hits something, the property described is about its straight travel. Option D is incorrect because opaque objects block light entirely, which is not what allows the path to become visible.