Explanation:

- Impure: Lacking purity, mixed with foreign matter or substances.

- Option 1 - Contaminated: This means that something has been made impure or unclean by coming into contact with a harmful or undesired substance.

- Option 2 - Segregated: Refers to something that has been separated or divided from others, not typically associated with purity or impurity.

- Option 3 - Sterile: Means free from bacteria or other living microorganisms, often used figuratively to mean "pure" or "untainted."

- Option 4 - Concentrated: Describes something that has been made denser or more intense, unrelated to purity.

Given these explanations, the most appropriate antonym for "impure" is:

- Option 3 - Sterile