The triangular sign indicates that you must...

The triangular sign in the question indicates the obligation for every driver to give way at the next intersection to vehicles travelling on the road being approached or to the lane they intend to join, both from the right and from the left, when necessary. Answer a), stop the vehicle and give way, would apply in the case of a stop sign. The stop sign requires a mandatory stop. It is the obligation of every driver to stop their vehicle at the next stop line or, if there is none, immediately before the intersection, and to give way at it to vehicles travelling on the road being approached. The difference between give way and stop is that with give way it is not necessary to stop if no vehicles are coming, whereas with stop you must always stop.