What must be assured when sealing an underground mine entrance?

The correct answer emphasizes the importance of providing access for re-entry through an air-lock when sealing an underground mine entrance. This is crucial because underground mines can pose hazards due to potential gas buildup, collapsing structures, or other dangers. An air-lock allows for controlled access to minimize the risk of exposing personnel to such hazards while enabling future inspections or emergency responses if necessary.

Ensuring an effective sealing process involves not just sealing off the mine for safety and environmental reasons, but also planning for potential re-entry scenarios. An air-lock serves as a barrier that can separate the sealed mine atmosphere from the surface, which is essential in managing potential risks. It allows for safe passage for qualified personnel during emergencies or for ongoing monitoring.

Other options, while potentially relevant in certain scenarios, do not capture the fundamental requirement of facilitating safe re-entry through a controlled environment. The necessity for a monitoring system, designated guard, or accessibility for emergency services, while important in broader mine safety context, does not specifically address the essential feature of an air-lock in sealing underground mine entrances effectively.