When preparing for a turn on a highway, you must signal your turn:

Answer: C — At least 500 feet before making the turn

Signaling at least 500 feet before making a turn on a highway is necessary to provide adequate notice to other drivers. This distance allows other vehicles to react appropriately, ensuring safety as everyone adjusts their speed and positioning in anticipation of the maneuver. Highways are typically faster-moving environments, and signaling well in advance helps prevent potential collisions and misunderstandings. It also reinforces the importance of clear communication on the road, especially in busy traffic conditions. The other options do not provide enough distance for effective signaling in a high-speed scenario. Signaling right before the turn or immediately as you start to turn does not give other drivers sufficient time to respond, possibly leading to dangerous situations. While signaling at least 100 feet before the turn is an improvement over the immediate options, it still may not be adequate on a highway where speeds are higher and more time is needed for others to react.