Answer:
0.9
Explanation:
The pka represents the force by which the molecules need to dissociate for the acids ,
Hence , lower the pka stronger will be the acid and that therefore will  dissociate completely and vice versa , for a weak acid higher the pka .
And in case of a base , it will be completely reversed , lower pKa , weaker base ,
and higher pKa , stronger base .
From the data of the question ,
0.9 is the lowest value of the pKa , hence , weakest base .
