Which word refers to making something impure by adding a foreign element?

The term "adulterate" specifically denotes the act of making something impure or of poorer quality by adding an inferior substance or foreign element. This word is often used in contexts related to food and substances, where the integrity of an item can be compromised through the addition of contaminants or low-quality ingredients.

For example, when a high-grade product is diluted with a cheaper additive to increase quantity or reduce costs, it is said to be adulterated. This not only affects the quality but can also have health implications if harmful substances are introduced.

In contrast, "amalgamate" refers to blending or combining different elements into a unified whole without the connotation of impurity. "Alter" simply means to change something without the specific context of reducing its quality. Therefore, "adulterate" is the precise term for the action of making something impure by incorporating foreign elements, which is why it is the correct answer.