Dormant volcanoes c. could still erupt but have not been active for a very long time.
Volcanoes that are quiet (inactive) for good amount of years, but are considered active, which could erupt sometime in the future are known as Dormant Volcanoes.
For example, Mount Fuji is inactive since its last eruption in 1707. Few volcanoes are dormant for a very long time but they erupting will be a surprise. For example, Mount Vesuvius before 79 AD.
Such dormant volcanoes are extremely dangerous as we do not know when this could potentially erupt. The volcanic eruption causes extreme heat of lava, suffocating ashes, and rocks pelting through the air.
