Final answer:
A lighthouse casts a straight shadow on the beach and is visible during both day and night, with its light serving as a navigational aid especially at night.
Step-by-step explanation:
The question asks whether a lighthouse x miles off the coast of a straight infinitely long beach casts a particular type of shadow on the beach and about its visibility conditions. The correct answer is: b) Casts a straight shadow on the beach.
Lighthouses are structured to cast light beams straight out towards the sea to serve as a navigational aid for ships, particularly during nighttime and in poor visibility conditions. Since light travels in a straight line unless obstructed or refracted, the shadow cast would also be straight. A lighthouse is generally visible both during the day and during the night, although its light is especially important and effective at night or during tumultuous weather conditions.