What signal indicates it is okay to open the MCD after arrival at the gate?

The signal that indicates it is acceptable to open the Main Cabin Door (MCD) after arrival at the gate is the one hi chime along with the extinguished seat belt sign. The hi chime serves as a notification to the cabin crew that the aircraft has safely arrived at the destination and that it is now safe for passengers to begin disembarking. The extinguishing of the seat belt sign signals that the crew can start preparing the cabin for passenger unloading.

This combination clearly communicates that the necessary safety protocols are in place, ensuring that the cabin is ready for an orderly and safe exit for passengers. Other options do not meet the criteria for indicating readiness to open the MCD; they either signify different operational states or are related to other safety measures that do not directly correlate with the opening of the door.