Which of the following will decrease CO bond character?

[palaphys]

Homework Statement

A carbonyl complex of Iridium (electronic configuration- [Xe] 4f14 5d7
6s2
) with the formula
IrCl(CO)(PPh3)2 is known as Vaska’s complex. Which of the following ligand substitution/s will
decrease the triple bond character of CO in Vaska’s complex

Relevant Equations

none

My attempt: If the triple bond character of CO is to reduce, then the extent of synergic bonding(backbonding) should increase. so with that logic, the groups to be introduced newly must have an electron donating nature. However, this is a single correct question and I think it can be both (a) and (b).
answer is given as (a). can somebody explain?