Which of the following compounds is used as an antiknock agent?

Among the given complexes that possess $CO$ ligand bridges are:
$\underset{I}{[Co_2(CO)_8]} \quad \underset{II}{[Fe_3(CO)_{12}]} \quad \underset{III}{[Mn_2(CO)_{10}]} \quad \underset{IV}{[Fe_2(CO)_9]}$

Which of the following substances is used as an antiknock compound?

What is the nature of the $Fe-C$ bond in $Fe(CO)_5$?

The $\pi$-acid ligand which uses its $d$-orbital during synergic bonding in its complex compound is

Which of the following organometallic compounds possesses both $\sigma$ and $\pi$ bonds?