The largest volcano on Earth, in terms of its mass and footprint, is considered to be Tamu Massif. While the other options mentioned in the question are significant volcanoes, they do not compare in size to Tamu Massif.

Option 1, Ojos del Salado, is a high stratovolcano located in the Andes Mountains, but it is not as large as Tamu Massif.

Option 2, Mount Vesuvius, is famous for its eruption in 79 AD that destroyed the city of Pompeii. However, it is a relatively small volcano compared to Tamu Massif.

Option 4, Mauna Kea, is a dormant volcano located in Hawaii. While it is known for its height and is the tallest mountain on Earth when measured from its base on the ocean floor, it does not have the same mass as Tamu Massif.

Therefore, the correct answer is option 3, Tamu Massif, as it is considered the largest volcano on Earth in terms of its mass and footprint.