A pot of water on a stove is heated to a rapid boil. what is the the gas inside the bubbles that form in the boiling water?

Answer is: gas inside the bubbles that form in the boiling water is most water vapor.

Water vapor bubbles start to form on nucleation sites. The water vapor bubbles start out larger as the temperature gets higher because more liquid is being converted to gas. Vapor-filled bubbles shrink and disappear completely.