The correct answer is option 4: water vapor.

When pure water is heated to its boiling point, it undergoes a phase change from a liquid to a gas, a process known as vaporization. During vaporization, water molecules gain enough energy to break away from the liquid and escape into the air as water vapor.

The bubbles that form and rise to the surface during boiling are composed primarily of water vapor. These bubbles are formed due to the high temperatures and the pressure created by the heat source, causing the water molecules to move faster and escape from the liquid.

Options 1 and 2 can be ruled out because air and hydrogen are gases that are not typically present in pure water. Option 3 can also be ruled out because although water is composed of hydrogen and oxygen, during boiling, the water molecules dissociate and primarily exist as water vapor.

Therefore, the correct answer is option 4: water vapor.