Answer

`P-c,f; Q-c,f: R-a,b; S-b-f`

Explanation

(P) NO - Neutral , Colorless gas
(Q) `N_2O` - Neutral Cololess gass
(R) `N_2O_3` - Acidic , blue colored
(S) `N_2O_4` - dimer of `NO_2` - Acidic , colorless compound