The correct answer is - a convergent boundary, where at least one of the plates is oceanic.
Volcanoes can be formed in multiple types of interactions between the tectonic plates, but most often, they form where there's a convergent boundary between a continental and oceanic tectonic plate. This can easily be seen by the famous ''Ring of Fire'', which is on and around the borders of the Pacific Ocean, and that is due to the heavier Pacific plate subducting beneath the lighter continental plates around it, thus creating a very long chain of volcanoes.
Quite simply, word of mouth and then deep faith in those that preached.
