Hydrogenation is a chemical reaction where hydrogen is added across the carbon-carbon double bonds present in alkenes. It`s used to convert unsaturated fats or oils to saturated fats or oils. This process requires a catalyst to proceed effectively.

Option 1, Copper: While copper is a catalyst, it is typically employed in the hydrogenation of aromatic rings or triple bonds, but not in the hydrogenation of alkenes.

Option 2, Zinc: Zinc is a reducing agent commonly used in organic synthesis but it`s not a catalyst for the hydrogenation process.

Option 3, Aluminum: Aluminum is not a catalyst for hydrogenation. It`s more commonly used in the making of aluminum alloys or in the Hall-Héroult process for producing aluminum.

Option 4, Nickel: Nickel, often in the form of Raney nickel, is a common catalyst for the hydrogenation of alkenes. Under high pressure and temperature, it can break the hydrogen molecules into atoms, which can then react with the double bond, adding hydrogen across it.

So, the correct answer is nickel.