1. Identify the species

The given hydrides belong to group 15:

$\mathrm{NH_3,\ PH_3,\ SbH_3,\ BiH_3}$

2. Relevant trend in group 15 hydrides

As we move down the group from nitrogen to bismuth:

• Atomic size increases
• The $E-H$ bond strength decreases
• The hydrides become less stable
• Hence they more readily donate hydrogen / get oxidized
• Therefore, reducing character increases down the group

So the reducing power follows:

$\mathrm{NH_3 < PH_3 < SbH_3 < BiH_3}$

3. Reasoning in terms of bond strength

A reducing agent is one that is easily oxidized. For these hydrides, weaker $E-H$ bonds mean the compound is more unstable and can act as a stronger reducing agent.

Since $\mathrm{Bi-H}$ bond is the weakest among the given options, $\mathrm{BiH_3}$ is the strongest reducing agent.

4. Evaluate options

• A: $\mathrm{SbH_3}$ — strong reducing agent, but less than $\mathrm{BiH_3}$
• B: $\mathrm{NH_3}$ — least reducing among these
• C: $\mathrm{BiH_3}$ — strongest reducing agent
• D: $\mathrm{PH_3}$ — stronger than $\mathrm{NH_3}$ but weaker than $\mathrm{SbH_3}$ and $\mathrm{BiH_3}$

5. Final answer

$\boxed{\mathrm{BiH_3}}$

So, the correct option is C.