Q.Why is BiH3 the strongest reducing agent amongst all the hydrides of Group 15 elements ?

Step 1 — Trend in atomic size.

Down Group 15, atomic radius increases: N < P < As < Sb < Bi.

Step 2 — Effect on bond enthalpy and stability.

Larger central atoms form longer, weaker E–H bonds with poorer orbital overlap, so bond dissociation enthalpy decreases down the group:

Step 3 — Reducing power.

A weaker, less stable hydride more readily releases hydrogen (acts as a hydrogen-atom/electron donor) to reduce other species. Since $BiH_3$ has the weakest bond and is the least thermally stable, it is the most reactive/easily decomposed and hence the strongest reducing agent of the series.

Track the atomic-size trend down the group, translate it into a bond-strength/thermal-stability order, and connect lowest stability directly to strongest reducing character.

• Don't confuse thermal stability with basicity trend — basicity of Group 15 hydrides actually decreases down the group (opposite direction reasoning applies to a different property), while reducing power increases down the group; keep the two trends separate.
• Don't say BiH₃ is simply "unstable" without linking it to the weak Bi–H bond dissociation enthalpy as the mechanistic cause.