What must a rowboat underway show if it does not have the specific lights required for a sailing vessel?

The requirement for a rowboat underway that does not have the specific lights for a sailing vessel is to exhibit a white light in sufficient time to prevent collision. This is crucial for safety as it allows other vessels to see the rowboat and recognize its presence, particularly in low-light conditions such as at dusk or at night.

The focus on the white light being shown "in sufficient time to prevent collision" emphasizes the importance of ensuring visibility to avoid accidents on the water. Rowboats, especially those not equipped with the standard navigation lights, must still adhere to safety practices which include signaling their presence clearly to other vessels. This guideline helps maintain safe navigation and communication among watercraft, thereby reducing the risk of incidents on the water.

While other options detail different configurations of lights that may be used by various types of vessels under different circumstances, the answer highlights that a rowboat’s primary obligation is to ensure it is visible with the simplest and most effective means—a white light—especially when lacking the more complex lighting arrangements of other types of vessels.