Correct Answer - A
`NH_(3)` is distinctly basic, `PH_(3)` is weakly basic, `AsH_3, SbH_3, and BiH_3,` do not show any basic property and therefore, are very weak bases. Nitrogen, because of its small size, has a lone pair of electrons concentrated on small region, so electron-density on small-sized nitrogen atom is more. Now as we go down the group, the size of central atom goes on increasing and, therefore, the same electronic charge is now distributed (or diffused) over a larger volume. As a result, electron-density decreases. Consequently, electron-donating ability of the hydrides to electron-deficient species decreases on moving down the group.
