Electron-withdrawing groups increase the acidic nature of the phenol. Nitro group is electron withdrawing group, hence it is the more acidic compound among the given phenols.  

Electron-donating groups decrease the acidic nature where methoxy group is more electron donating, hence less acidic than the iso-propyl group which donates electrons by hyperconjugation effect.

$\therefore$ Order of acidic strength is (c) > (a) > (d) > (b)

The order of pK_a values are (b) > (d) >(a) > (c).