Identifying Hydroxyl Functional Group

In organic chemistry, functional groups significantly influence the chemical properties and reactions of compounds. Consider the following four organic compounds:

1) 1-Butanol (C₄H₁₀O)
2) 2-Butanamine (C₄H₁₁N)
3) 3-Methylbutanoic acid (C₅H₁₀O₂)
4) Diethyl ether (C₆H₁₄O)

Which of these compounds contains a hydroxyl group (-OH) as its functional group?