Formula of ferrocene is :

Ferrocene is an organometallic chemical . Two cyclopentadienyl rings are bound on either side of an iron atom in the middle of the molecule. It is an orange solid that sublimes above room temperature and dissolves in most organic solvents. It smells like camphor.

Ferrocene $\to \mathrm{Fe}\left({\eta }^5-{\mathrm{C}}_5{\mathrm{H}}_5\right)$Hence correct option D.