Finely divided nickel is the catalyst — option 3.

The reaction. Vegetable oils are triglycerides of unsaturated fatty acids — their hydrocarbon chains contain carbon-carbon double bonds, which put kinks in the chain, prevent close packing, and keep the fat liquid at room temperature. Passing hydrogen through the oil at about 150 to 200 °C over finely divided nickel adds hydrogen across those double bonds :

\(-\text{CH}=\text{CH}-\ +\ \text{H}_{2}\ \xrightarrow{\ \text{Ni},\ 200\ ^\circ\text{C}\ }\ -\text{CH}_{2}-\text{CH}_{2}-\)

The chains straighten, pack closely, and the melting point rises — the oil becomes a semi-solid fat, vanaspati. Because the process makes an unsaturated fat behave like a saturated one, it is called hardening of oils.

Why nickel and not palladium. Palladium works, and works at lower temperature, but nickel is a fraction of the cost and is entirely adequate for a bulk commodity process — and cost is decisive when millions of tonnes are processed. Both are examples of heterogeneous catalysis: the solid catalyst provides a surface on which hydrogen molecules dissociate and meet the double bond, and it is filtered off and reused.

The health footnote. Partial hydrogenation converts some cis double bonds into the trans form, producing trans fats, which raise LDL cholesterol and lower HDL. India’s food regulator has progressively capped trans fats in edible oils and fats at 2 per cent, and industry has moved towards full hydrogenation and interesterification instead.

Hence, the answer is Ni.