Among the following metal carbonyls, the C – O bond order is lowest in

Carbon-oxygen bond order in ‘CO’ is three ;

In metal carbonyls C-O bond order will be between 2-3;

Due to back bonding M-C bond will have bond order of 1-2;

Back bonding is due to donation of electron pair from metal to carbon of ‘CO’ into its π* ABMO.

In option (4) the complex have negative charge, so backdonation by metal is high.

$\therefore$M-C bond order increases.

Ultimately C-O bond order decreases.