Identify the group 15 elements listed

Group 15 elements here are:

• $\mathrm{P}$
• $\mathrm{As}$
• $\mathrm{Sb}$
• $\mathrm{Bi}$

Find which of these is a metal

In group 15:

• $\mathrm{P}$ is a non-metal
• $\mathrm{As}$ is a metalloid
• $\mathrm{Sb}$ is a metalloid
• $\mathrm{Bi}$ is a metal

So the only metal among the given options is $\mathrm{Bi}$.

Check the hydrides of group 15

The corresponding hydrides are:

• $\mathrm{PH_3}$
• $\mathrm{AsH_3}$
• $\mathrm{SbH_3}$
• $\mathrm{BiH_3}$

Trend of reducing power in group 15 hydrides

Down the group, the $\mathrm{E-H}$ bond weakens because atomic size increases. Therefore, hydrides decompose more easily and their reducing character increases.

So, $\mathrm{NH_3 < PH_3 < AsH_3 < SbH_3 < BiH_3}$ in reducing power.

Hence, among the listed options, the hydride with the strongest reducing power is $\mathrm{BiH_3}$.

Conclusion

The group 15 element which is a metal and forms the hydride with strongest reducing power is: $\boxed{\mathrm{Bi}}$

Compare with stored correct answer

Stored correct answer: C

Option C corresponds to $\mathrm{Bi}$, which matches our derived answer.