The correct option is A It is formed of living cells.
Heartwood are also called duramen. It is dead, central wood of trees. It's cells usually contain tannins or other substances that make it dark in colour and sometimes aromatic. Heartwood is mechanically strong, resistant to decay, and less easily penetrated by wood-preservative chemicals than other types of wood. One or more layers of living and functional sapwood cells are periodically converted to heartwood. Thus, option A is correct.