The correct option is C Surface charge density on the 15 cm sphere will be greater than that on the 20 cm sphere
After redistribution, charges on them will be different, but they will acquire common potential
i.e. kQ1r1=kQ2r2⇒Q1Q2=r1r2
As σ=Q4πr2⇒σ1σ2=Q1Q2×r22r21⇒σ∝1r
i.e. surface charge density on smaller sphere will be more.