The correct answer is (c), nickel.
Nickel is a transition metal that is used as a catalyst in many chemical reactions, including the hydrogenation of vegetable oils. Hydrogenation is a process in which hydrogen is added to an unsaturated compound, such as a vegetable oil, to make it more saturated. This makes the oil more stable and less likely to go rancid.
Nickel is a good catalyst for hydrogenation because it has a high surface area and is able to absorb hydrogen atoms. It is also relatively inexpensive and easy to work with.
The other options are not as good catalysts for hydrogenation. Zinc is a metal that is not as active as nickel and is not as good at absorbing hydrogen atoms. Aluminium is a non-metal that is not a good catalyst for any chemical reaction. Iron is a metal that is a good catalyst for some chemical reactions, but it is not as good as nickel for hydrogenation.
In conclusion, the catalyst mainly used in hydrogenation of vegetable oils is nickel.