The correct answer is option 3: Addition reaction.

In the hydrogenation of vegetable oils, hydrogen gas (H2) is added to the unsaturated double bonds present in the molecules of vegetable oils. This addition of hydrogen gas to the carbon-carbon double bond results in the formation of a single bond and converts the unsaturated vegetable oil molecules into saturated fats.

This process is catalyzed by a nickel catalyst, which helps in promoting the reaction to occur at a faster rate. The addition of hydrogen to the vegetable oil molecules is a classic example of an addition reaction, where a molecule is added to a compound to form a new product.

Substitution reactions involve the substitution of one atom or group with another, but in the hydrogenation of vegetable oils, there is no substitution of any atom or group.

Elimination reactions involve the removal of a molecule or group from a compound to form a double bond or an unsaturated compound, which is not the case in the hydrogenation of vegetable oils.

Free-radical polymerization is a process in which monomers (small units) join together to form a polymer chain. It is not applicable to the hydrogenation of vegetable oils.

In conclusion, the hydrogenation of vegetable oils using a nickel catalyst is