Ans.
Beads on a string structure shows DNA molecule, which wraps around histone proteins. This structure is known as chromatin. The histone molecules are supposed as beads and DNA molecule represents a string, thus known as beads on string structure.
It plays an important role in DNA packaging as it folds very long molecules of DNA into a dense, compact form that can fit into a small nucleus or cell.
Hence, the correct answer is option d.