Water drops assume a spherical shape because of surface tension.

Surface tension is a property of liquids caused by the cohesive forces between the liquid molecules. Molecules at the surface are pulled inward more strongly, which causes the liquid surface to behave like a stretched elastic membrane and contract to the minimum possible surface area.

A. polarity is the reason for the strong cohesive forces (hydrogen bonds) in water molecules, which in turn causes surface tension, but surface tension is the direct physical property responsible for the spherical shape.

B. viscosity is a measure of a fluid's resistance to flow (its "thickness") and does not determine the shape of a drop.

C. density is the mass per unit volume of a substance. It plays a role in how gravity affects larger drops (making them less spherical), but it is not the reason for the fundamental spherical shape in the absence of external forces like gravity.

The correct answer is surface tension - option D