The correct answer is: D. Replacement reaction
A replacement reaction is a chemical reaction in which one element or group of elements is replaced by another element or group of elements in a compound.
In the hydrogenation of vegetable oil, hydrogen gas is added to vegetable oil in the presence of a catalyst, such as nickel. The hydrogen atoms replace some of the double bonds in the vegetable oil, making it more saturated and solid at room temperature.
Option A, displacement reaction, is incorrect because in a displacement reaction, one element is displaced from a compound by another element. For example, in the reaction of sodium metal with hydrochloric acid, the hydrogen atoms in the hydrochloric acid are displaced by the sodium atoms, forming sodium chloride and hydrogen gas.
Option B, mixture reaction, is incorrect because in a mixture reaction, two or more substances are mixed together but do not react chemically. For example, when you mix salt and water, you get a mixture of salt and water, but the salt does not react with the water.
Option C, combination reaction, is incorrect because in a combination reaction, two or more substances combine to form a new substance. For example, in the reaction of hydrogen and oxygen to form water, the hydrogen and oxygen atoms combine to form water molecules.