Phosphine on decomposition produces phosphorus and hydrogen as follows $4{\mathrm{PH}}_{3\left(\mathrm{g}\right)}\to {\mathrm{P}}_{4\left(\mathrm{s}\right)}+6{\mathrm{H}}_{2\left(\mathrm{g}\right)}$
When 100 ml of phosphine is decomposed the change in volume under laboratory conditions is (assume volume exerted by solids is neglected)

$4{\mathrm{PH}}_{3\left(\mathrm{g}\right)}\to {\mathrm{P}}_{4\left(\mathrm{s}\right)}+6{\mathrm{H}}_{2\left(\mathrm{g}\right)}$ 

(Elemental phosphorus exists as tetra-atomic molecules).

When the temperature and pressure remain the same, 4 ml of ${\mathrm{PH}}_3$ will give 6 ml of ${\mathrm{H}}_2$. 

 (On applying Gay-Lussac's law ) 

Hence, 100 ml of ${\mathrm{PH}}_3$ will yield 150 ml of ${\mathrm{H}}_2$.

Increase in volume = (150-100) ml = 50 ml. (The volume of solid phosphorus is negligible).