The question asks us to identify which option is NOT a mixture. Let`s analyze each option to determine if it is a mixture or not:

1. Air: Air is a mixture of several gases, including nitrogen, oxygen, carbon dioxide, and others. Therefore, option 1 is a mixture.

2. Mercury: Mercury is an element and not a mixture. It exists naturally as a pure substance. Therefore, option 2 is the correct answer.

3. Milk: Milk is a mixture that consists of water, fats, proteins, lactose, and various other substances. Therefore, option 3 is a mixture.

4. Cement: Cement is a mixture of minerals such as limestone, clay, and gypsum. It is mixed with water to form a solid mass. Therefore, option 4 is a mixture.

In conclusion, the only option that is not a mixture is option 2, which is mercury.