The correct answer is A. hydrogen.
Hydrogen is used in the manufacture of vanaspati from vegetable oil because it is a good reducing agent. It helps to break down the complex molecules in the vegetable oil into simpler molecules that can be used to make vanaspati.
Oxygen is not used in the manufacture of vanaspati because it is an oxidizing agent. It would actually help to break down the vegetable oil into even smaller molecules, which would not be desirable.
Nitrogen is not used in the manufacture of vanaspati because it is a relatively inert gas. It would not react with the vegetable oil and would not help to break it down.
Carbon dioxide is not used in the manufacture of vanaspati because it is a waste product of the process. It would not help to break down the vegetable oil and would actually make the process more difficult.
In conclusion, the gas used in the manufacture of vanaspati from vegetable oil is hydrogen.