The boiling points of the hydrides of 15-group elements are in the order:

NH3 molecules are associated by strong intermolecular H-bonds. As a result, its boiling point in exceptionally high. The intermolecular forces in PH3 are vander Waal's forces, due to which its boiling point in lower than NH3. In moving from PH3, to BiH3, boiling points increase. This is due to the increase in the magnitude of vander Waal's forces, owing to the increase in the molecular size (or increase in molar mass). The vander Waal's forces in SbH3, become stronger than intermolecular H-bonds in NH3. As a result, boiling point of SbH3, becomes more than that of NH3. Hence, the correct sequence of boiling points is  PH3