Q.Which one of the following is the natural nanomaterial ?

Nanoscience is the study of structures and materials on the scale of nanometres ($1\,\text{nm}=10^{-9}\,\text{m}$); nanotechnology is the associated technology of designing, producing, characterizing and applying nano-structured materials. A solid whose particle size is under $100\,\text{nm}$ is a nano solid; above that it is a bulk solid -- the two can share identical chemical composition (e.g. ZnO exists in both forms) yet differ strikingly in mechanical, electrical, optical and magnetic properties, governed at the nanoscale by quantum confinement and surface effects. …