The correct option is B 4.74
Since , pH+pOH=14pOH=14−pH
pOH=2.57
The aqueous solution of NH3 forms NH4OH which is a weak base. The formula of pOH for a weak base is :
pOH=12pKb−12logCor, 2.57×2=pKb−logC
or,5.14=pKb−log0.4
or,5.14=pKb−(2log2−log10)pKb=4.74