The correct answer is option 2, sugar.

A compound is a substance that is made up of two or more elements chemically combined. In the case of sugar, it is a compound called sucrose which is made up of the elements carbon, hydrogen, and oxygen.

Option 1, milk, is not a compound because it is a mixture of various substances such as water, proteins, fats, and carbohydrates.

Option 3, tea, is not a compound either. It is a mixture of water and various compounds found in tea leaves such as caffeine, flavonoids, and tannins.

Option 4, coffee, is also not a compound. It is a mixture of water and various compounds found in coffee beans such as caffeine, acids, and oils.

Therefore, the only option that fits the definition of a compound is option 2, sugar.