$N H_{3} > P H_{3} > A S H_{3} > S b H_{3}$

$N H_{3}$ is the most basic because of its small size, the electron density of electron pair is concentrated over small region. As the size increases, the electron density gets diffused over a large surface area. Hence, the ability to donate the electron pair (basicity) decreases.

The other three statements given are correct.