The correct answer is option 3: solid carbon dioxide.

Dry ice is the solid form of carbon dioxide. It gets its name because it does not melt into a liquid state when heated, but rather sublimates directly from a solid to a gas. This makes it useful for creating very cold temperatures in laboratory experiments.

Option 1, gaseous carbon dioxide, is incorrect because it is not in the form of dry ice. Gaseous carbon dioxide is the result of heating or releasing pressure from the solid or liquid form.

Option 2, liquid carbon dioxide, is also incorrect because it is not in the form of dry ice. Liquid carbon dioxide is obtained by compressing and cooling gaseous carbon dioxide.

Option 4, solid hydrogen oxide, is incorrect as well. Solid hydrogen oxide is another term for ice, which is made of water molecules.

In conclusion, dry ice is the solid form of carbon dioxide, making option 3 the correct answer.