The correct answer is (a) Hydrogen.
Hydrogen is used in the manufacture of Vanaspati Ghee from Vanaspati Oil because it is a reducing agent. This means that it can remove oxygen from other molecules, which is necessary to convert Vanaspati Oil into Vanaspati Ghee.
Oxygen is not used in the manufacture of Vanaspati Ghee because it is an oxidizing agent. This means that it can add oxygen to other molecules, which would not be desirable in this process.
Nitrogen is not used in the manufacture of Vanaspati Ghee because it is a neutral gas. It does not react with other molecules, so it would not be helpful in this process.
Carbon dioxide is not used in the manufacture of Vanaspati Ghee because it is a waste product of the process. It is not necessary for the process to occur, and it would only add to the cost of production.