a) 
As 50=5X5X2=
=Denominator has prime factors 2 and 5
Decimal representation of
will be terminating
b) 
As 32=
=Denominator has 2 as a prime factor.
Decimal representation of
will be terminating.
c) 
As 90=2X3X3X5
=Denominator has prime factors other than 2 and 5.
Decimal representation of
will not be terminating.