Arrange the following phenols in increasing order of pKa, value.
(a)I<Il<III  (b)III<l<II  (c)III<II<I  (d)I<III< II

Presence of more electron withdrawing  groups(EWGs) in the ring will make it more stronger acid.Aromatic ring is more affected by the presence of groups on the ortho and para position.hence look for EWGs on ortho or para position.
ortho nitrophenol is less acidic than para nitrophenol due to intramolecular hydrogen bonding which makes this hydrogen less ionizable  and hence less acidic .
So acidity order is :
meta -nitrophenol < ortho -nitrophenol<para - nitrophenol
Higher is the acidity ... more is the Ka value .... less is the pKa value
so pKa  order is :
(a)I<Il<III