The question asks us to specify the effect of introducing a functional group into a carbon chain.
A simple carbon chain, with only carbons and hydrogens, doesn't show great reactivity because the carbon-to-carbon and carbon-to-hydrogen bonds are strong and the charge of the electrons from theses bonds is spread relatively even over the bonded atoms. Therefore, chains that contain only these two atoms, specially those with only single bonds, are not very reactive.
On the other side, if one or more weaker bonds are introduced into the chain, with the charge of electrons unequally spread over the bond, the reactivity of the molecule tends to increase. When an heteroatom is introduced, for example, the electronegativity difference between atoms will make the electrons more drawn to one side of the bond than the other.
For that reason, we can say that a functional group introduces heteroatoms into a carbon chain to increase the reactivity of this chain (option A).