Which type of sign denotes an entrance to a runway or a critical area?

Answer: A — Mandatory Signs

Red-background mandatory signs indicate you must not enter or proceed beyond them without explicit clearance. They mark the boundary to runways and other critical areas, so you are required to stop and obtain authorization before crossing. This serves as a clear, non-negotiable instruction to protect access to sensitive surfaces. Informational signs simply provide directions or information, and location signs show where you are on the field; neither denotes an entrance boundary. Approach signs aren’t a standard category for marking runway or critical-area entrances.