(a)   I < II < III (b) III < I < II (c) III < II < I (d) II < III < I

pKa = -logKa
Higher Ka means less pKa value.
A high Ka value also means strong acid and weak base or
A low Ka value also means  weak acid and  strong base.
Here we can compare the basicity of N atoms with one lone pair in all structures.
Remember more is the delocalization of lone pair of nitrogen atom due to resonance ,less is the basicity and  more Ka and less pka .
Maximum delocalization is with three phenyl rings and minimum with one phenyl ring.