Identify the trend in group-15 hydrides

The given hydrides are: $PH_3,\, AsH_3,\, SbH_3,\, BiH_3$

These are hydrides of group-15 elements.

Reducing character of hydrides

A hydride acts as a reducing agent when it can easily donate hydrogen/electrons and get oxidized itself.

Down the group, the bond strength of the $E-H$ bond ($E = P, As, Sb, Bi$) decreases because the size of the central atom increases.

So, $P-H > As-H > Sb-H > Bi-H$ in bond strength.

Relation between bond strength and reducing nature

Weaker $E-H$ bond means the hydride decomposes more easily and can act as a stronger reducing agent.

Therefore, reducing character increases down the group: $PH_3 < AsH_3 < SbH_3 < BiH_3$

Evaluate the options

• A: $AsH_3$ — not the strongest
• B: $BiH_3$ — strongest reducing agent
• C: $PH_3$ — weakest among these
• D: $SbH_3$ — strong, but less than $BiH_3$

Final answer

Hence, the strongest reducing agent is: $\boxed{BiH_3}$