For an ideal gas, number of moles per litre in terms of its pressure P, gas contant R and
temperature T is
(a) PT/R (b) PRT (c) P/RT (d) RT/P

@ satendra 
if u can recall the ideal gas equation that is 
Pv= nRT, in which 
p =- pressure of 1 mole of gas 
v = volume , n= no of moles , 
R= gas conatant  t = temp of the substances .. 
so, by taking volume as a constant , 
u can get the n as 
n= P/RT 
so, option c turns out to be the correct answer .. 
HOPE IT CLEARS YOUR DOUBT ..