Which of the following is NOT a requirement for street-legal motorcycles in South Carolina?

In South Carolina, having a valid insurance policy is indeed a critical requirement for street-legal motorcycles. While it may seem like a basic requirement for all vehicles, the emphasis here is on the other options related to motorcycle equipment.

Motorcycles must be equipped with a working turn signal light, as these are essential for indicating turns and lane changes to other drivers, thus enhancing safety on the road. Additionally, having at least one rearview mirror is necessary for the operator to maintain awareness of their surroundings, especially for vehicles approaching from behind. Finally, a functioning horn is required to alert other road users to the motorcycle's presence, especially in situations where visibility may be compromised or to signal for attention.

Therefore, being insured is a fundamental requirement that does apply but is not specific to the motorcycle’s equipment or features, which is what the other items are focused on. This highlights the distinction between legal operating requirements related to insurance and the specific mechanical or safety features necessary for the motorcycle itself to be street-legal.