Synergic bonding in metal carbonyl complexes [M = Metal]:

The correct options are
A Decreases the $C-O$ bond strength
B Increases the $M-C$ bond strength
C Increases the bond order for $M-C$ bond
The effect of synergic bonding is as follows:
It strengthens the $M-C$ bond while weakening the $C-O$ bond.

This back bonding or synergic bonding shortens the $M-C$ bond length while increasing the $C-O$ bond length.

It does not change the geometry of the complex.