Which of the following is not a functional group?

Hydrogens in hydrocarbon chains are replaced by other elements in such a way that the valency of the carbon remains satisfied. The elements which replace hydrogen are known as heteroatoms.These heteroatoms and the groups which contain them give a specific type of properties to a compound, these groups are known as functional groups. Functional groups include aldehydes, ketones, alcohol, carboxylic acids etc.