The correct option is C r1/r2
According to Gravitational Law
F=GM1M2r2 ......(i)
Where M1 is mass of planet & M2 is the mass of any body. Now according to Newton's second law, a body of mass M2 feels gravitational acceleration g which is
F=M2g ......(ii)
So from (i) & (ii), we get g=GM1r2
So the ratio of gravitational acceleration due to two planets is
g1g2=M1r21×r22M2=(4/3)πr31×ρr21×r22(4/3)πr32×ρ
g1g2=[r1r2] (both planets have same material, so density is same)