Which of the following statements about GRT is correct?

Gross Registered Tonnage (GRT) is a measurement that is used to estimate the amount of usable space available for revenue-generating activities on a ship, such as passenger accommodations, cargo holds, and other commercial areas. It represents the total internal volume of a vessel and is a crucial metric for understanding the ship's capacity for revenue-generating purposes. By focusing on the usable areas of the ship, GRT helps in assessing how many passengers or how much cargo the vessel can accommodate, which is essential for both operational planning and regulatory compliance.

The other options relate to aspects of maritime measurements but do not accurately describe GRT. For instance, while it does involve the overall size of the ship in a broad sense, it is more specifically about the internal volume rather than just the overall measurements of the vessel. GRT is not focused on legal length requirements or the volume of fuel tanks, which are metrics governed by different standards in maritime regulations. Thus, the emphasis on usable space for revenue specifically pinpoints why this option stands out as the correct one.