The correct option is A drdt+K=0
The rate of evaporation is measured by the rate of change of volume ofthe spherical droplet.So if at an instant of time the radius is r Surface area = 4πr2The rate of evaporation=dVdtwhere V=Volume of the dropletdVdt∝4πr2dVdt=−K(4πr2)Here the minus sign is therebecause the the volume is decreasing.V(Volume of sphere)=43πr3dVdt=ddt(43πr3) =43π×3r2drdt =4πr2drdtSo, 4πr2drdt=−K( 4πr2) drdt+K=0