The most appropriate antonym of the given word "pure" is:

Option 4: Indecent

Explanation: The word "pure" refers to something that is clean, untainted, or free from impurities. Its antonym should convey the opposite meaning, indicating impurity, contamination, or something that is morally or socially unacceptable. Option 4, "indecent," fits this description as it means lacking modesty, morality, or propriety. Therefore, option 4 is the most appropriate antonym for "pure" in this context.