Which of the following statements is true regarding OBQ?

The correct statement about OBQ, which stands for "Onboard Quantity," is that it refers to quantities of liquid only. In the context of marine operations and tankerman duties, OBQ specifically pertains to the liquid cargo present in the vessel's tanks at a particular time, typically at the start of loading operations. This is crucial for ensuring accurate load calculations, cargo management, and confirming the amount of cargo onboard before departure.

This statement is accurate since OBQ does not typically account for additional materials like sediments, which would fall under different definitions and measurements, such as "total volume on board" or "sounded volumes." The other options that suggest OBQ includes sediments, is only measured after discharging operations, or pertains to whole volumes at the time of loading, do not align with the established definitions and practices of maritime cargo handling and measurement.