What is required for a vessel to be classified as a motor vessel?

To classify a vessel as a motor vessel, it is essential that it is propelled by machinery. This definition highlights the primary characteristic that distinguishes motor vessels from sailing vessels and other types of craft that may rely on wind or human power for propulsion. The mechanical propulsion can come from various types of machinery, such as outboard motors, inboard engines, or other similar systems designed to provide the necessary thrust for movement through the water.

Other options do not align with the criteria for classifying a motor vessel. For instance, the requirement regarding gross tonnage pertains to regulatory aspects and does not define a vessel as a motor vessel. Additionally, having a sail is indicative of a sailing vessel rather than a motor vessel. Lastly, while registration with the Coast Guard is important for legal and safety purposes, it does not alter the nature of propulsion that defines a motor vessel. Thus, option C correctly emphasizes the significance of machinery in classification.