The purpose of runway hold markings across a holding bay is to?

The purpose of runway hold markings across a holding bay is to hold aircraft in a particular part of the holding bay. These markings are specifically designed to provide visual guidance to pilots, indicating where they should stop their aircraft while waiting for clearance to proceed onto the runway. This ensures that no aircraft inadvertently cross into the runway area without proper authorization, thereby maintaining safety and preventing potential runway incursions.

Option C captures this function accurately, as it reflects the need to manage aircraft positioning in relation to active runways, ensuring that they are held safely at a designated point until they are cleared for takeoff or landing. This is crucial in busy airspace where multiple aircraft may be operating simultaneously. The other options reference prohibitions, designations of taxiways, and markings that denote runway endpoints, which do not accurately describe the specific function of hold markings in the context of runway safety and traffic management.