b
In group \(15\) hydrides, the basic character decreases on going down the group due to decrease in the availability of the lone pair of electrons because of the increase in size of elements from \(\mathrm{N}\) to \(Bi\). Thus, correct order of basicity is \(\mathrm{NH}_{3}>\) \(\mathrm{PH}_{3}>\mathrm{AsH}_{3}>\mathrm{SbH}_{3}\)