The question asks which property makes a free water drop take up a round shape.
Viscosity is about how easily a liquid flows, it does not pull the drop into a sphere.
Density only tells us mass per unit volume, it says nothing about drop shape.
Polarity explains attraction between water molecules in general but is not the direct cause of the round shape.
Surface tension acts like a skin on the liquid surface and always tries to shrink the surface area to a minimum. For a given volume, a sphere gives the smallest surface area of all shapes, so the drop becomes spherical.