Expressing power of any machine can be defined as the maximum number of languages it can accept..
if machine M1 can accept more languages than M2 , then we can say that expressing power of M1 is greater then M2.
In this case languages accepted by NPDA is more then DPDA ,
( DPDA accepts proper subsets of the class of languages accepted by NPDA ) .
so expressing power of NPDA is more then DPDA.
Same kind of gate question :
https://gateoverflow.in/2110/gate2011_8