Identifying Dominant Functional Groups in Organic Compounds

In organic chemistry, functional groups are specific groups of atoms that confer characteristic properties to the molecules they are part of. Understanding functional groups is crucial for predicting the reactivity and interactions of organic compounds. Consider the following compounds:

1. 2-Butanol

2. Hexanoic acid

3. Styrene

4. Acetonitrile

Based on the structural formulas of these compounds, which functional group is present in the greatest number of these molecules?