The correct option is D 4.85
Given, Kh=4×10−10,[NH4Cl]=0.5 M
NH4Cl is a mixture of strong acid and weak base
we know the relation
Kh=KwKb
4×10−10=10−14Kb
Kb=10−144×10−10
Kb=2.5×10−5
pKb=−log10Kb
=−log (2.5×10−5)
=6−log 25
=6−2×log5
=4.6
Also, pH=12×(pKw−pKb−log C)
pH=12×[14−4.6−log 0.5]
pH=12×[14−4.6+(1−0.7)]
pH=4.85