A reducing agent is a compound or element that losses electron.

A strong reducing agent looses electron more easily as compared to weak reducing agent.

SbH₃ acts as strongest reducing agent due to minimum value of bond dissociation enthalpy.

It requires less bond dissociation enthalpy to break bonds, therefore it can easily donate electrons to other chemical species.

NOTE: lesser will be bond dissociation enthalpy, less time is taken to break the bond between atom and more easily it can loses electrons to form bond with another atom

Therefore stronger is the reducing agent.

Hence the correct option is (iv)