What is the safe way to drive on a road where this safety sign is installed?

Answer: ③ Vehicles already circulating have priority over vehicles entering.

This sign (No. 109) warns of a roundabout ahead. Vehicles already in the roundabout have priority, so entering vehicles must drive slowly or stop temporarily and yield. Traffic circulates counterclockwise, and both parking and stopping are prohibited inside. [Enforcement Rule of the Road Traffic Act, Annex 6; Road Traffic Act, Art. 25-2]