As we move down the group the size of the element increases and therefore, the length of the E-H bond increases and its strength decreases. In other words, as we move down the group, the E-H bond can break more easil to evolve `H_(2)` gas which acts as the reducing agent. Thus, Bi-H bond is the weakest amongst the hydrides of elements of group 15 and hence `BiH_(3)` is the strongest reducing agent.
