Complex compounds:
Complex compounds are substances that contain a central metal atom or ion surrounded by ligands. Ligands are neutral molecules or ions that donate pairs of electrons to the metal atom or ion, forming coordinate covalent bonds. These bonds are also known as dative bonds.

Ferrocene:
Ferrocene is the correct answer because it is a complex compound. It consists of a central iron atom bonded to two cyclopentadienyl rings. Each cyclopentadienyl ring acts as a ligand, donating its five π-electrons to the iron atom. This forms a sandwich-like structure, with the iron atom sandwiched between the two cyclopentadienyl rings.

Trimethyl aluminium:
Trimethyl aluminium is not a complex compound. It is an example of an organometallic compound, which contains a direct bond between a metal atom and a carbon atom. In this case, aluminium is bonded to three methyl groups. There are no ligands present in trimethyl aluminium.

Diethyl zinc:
Diethyl zinc is also not a complex compound. It is another example of an organometallic compound, with zinc bonded to two ethyl groups. Like trimethyl aluminium, there are no ligands present in diethyl zinc.

Nickel tetra carbonyl:
Nickel tetra carbonyl is a complex compound. It consists of a central nickel atom bonded to four carbon monoxide (CO) ligands. Each CO ligand donates a pair of electrons to the nickel atom, forming coordinate covalent bonds.

In summary, out of the given options, only ferrocene and nickel tetra carbonyl are complex compounds. Ferrocene is a sandwich-like complex with a central iron atom and two cyclopentadienyl ligands, while nickel tetra carbonyl has a central nickel atom bonded to four carbon monoxide ligands.