Which volcano is known as the southernmost active volcano in the world?

Mount Erebus is recognized as the southernmost active volcano in the world, located on Ross Island in Antarctica. It is unique not only for its geographical position but also for its continuous activity, which includes an active lava lake. The extremely cold environment of Antarctica makes Mount Erebus a distinctive site for volcanic activity, as most volcanic activity is typically found in warmer climates.

In contrast, the other options listed are located in different regions: Mount Vesuvius and Mount Etna are both in Italy, while Mount St. Helens is located in the United States. These volcanoes, while notable for their eruptions and historical significance, are not situated as far south as Mount Erebus, substantiating why it stands out as the southernmost active volcano.