The strongest reducing agent amont the following is:NH3SbH3BiH3PH3

The reducing power of a compound is determined by its ability to donate electrons. In the case of hydrides, the reducing power increases as we move down the group in the periodic table. This is because the size of the atom increases, which makes the bond between the hydrogen and the atom weaker, thus making it easier to donate the hydrogen (which is the same as donating an electron).

The order of the elements given (NH3, SbH3, BiH3, PH3) in the periodic table is N (Nitrogen), P (Phosphorus), Sb (Antimony), and Bi (Bismuth). Nitrogen is in period 2, Phosphorus is in period 3, Antimony is in period 5, and Bismuth is in period 6.

Therefore, based on the periodic table, the strongest reducing agent among NH3, SbH3, BiH3, and PH3 would be BiH3, as Bismuth is the furthest down the group.