Which of the following gases is consistently seen to be most abundant in a volcanic eruption?

Water vapour

The correct answer is option 3. Water vapour.

Water vapor (water in its gaseous state) is consistently seen to be most abundant in volcanic eruptions. When magma rises to the surface during an eruption, it often contains a significant amount of water dissolved in it. As the magma reaches the Earth's surface and pressure decreases, the dissolved water vaporizes, producing steam. This steam, along with other volcanic gases, is expelled during the eruption.

While other gases such as sulfur dioxide (option 1), carbon dioxide (option 2), and hydrogen sulfide (option 4) are also commonly emitted during volcanic eruptions, water vapor tends to be the most abundant gas present.