The strongest reducing agent among the following is:

The hydrides of group 15 elements are ammonia ($$NH_3$$), phosphine ($$PH_3$$), arsine ($$AsH_3$$), stibine ($$SbH_3$$), and bismuthine ($$BiH_3$$).

The reducing power of these hydrides increases down the group. This is because the stability of the hydrides decreases down the group due to the decrease in bond strength of the E-H bond (where E is the group 15 element). As the size of the central atom increases down the group, the bond length increases and the bond strength decreases, making it easier to break the E-H bond and release hydrogen gas, which acts as a reducing agent.

The order of reducing power is: $$NH_3 < PH_3 < AsH_3 < SbH_3 < BiH_3$$

Among the given options:

• A. $$NH_3$$ (ammonia) is the weakest reducing agent.
• D. $$PH_3$$ (phosphine) is stronger than $$NH_3$$ but weaker than the others.
• B. $$SbH_3$$ (stibine) is stronger than $$PH_3$$ but weaker than $$BiH_3$$.
• C. $$BiH_3$$ (bismuthine) is the strongest reducing agent because bismuth is the largest atom in the group.

Therefore, the strongest reducing agent is $$BiH_3$$, which corresponds to option C.

The correct answer is C. $$BiH_3$$.