Which is the correct formula of the Wilkinson catalyst?

Which of the following has the longest $C-O$ bond length? (Free $C-O$ bond length in $CO$ is $1.128 \ \mathring{A}$.)

Which one is an organometallic compound?

The correct statements about metal carbonyls are:
$A$. The metal-carbon bonds in metal carbonyls possess both $\sigma$ and $\pi$-character.
$B$. Due to synergic bonding interactions between metal and $CO$ ligand, the metal-carbon bond becomes weak.
$C$. The metal-carbon $\sigma$ bond is formed by the donation of a lone pair of electrons on the carbonyl carbon into a vacant orbital of metal.
$D$. The metal-carbon $\pi$ bond is formed by the donation of electrons from a filled $d$-orbital of metal into a vacant $\pi^*$ orbital of $CO$.
Choose the correct answer from the options given below:

What type of bonding is present in the $Fe-C$ bond of $Fe(CO)_5$?

The structure of ferrocene is $Fe(\eta ^5 - C_5H_5)_2$.