I think the correct answer would be strong gravity. The factor that allows the earth to hold onto water would be the strong gravitational pull of the earth. In fact, it is what holds everything onto earth not only water. Hope this answers the question.
Gravity or gravitational force is the downward force or pull exerted by the Earth on the matter present on its surface. The gravitational force of the Earth holds onto all the matter that is present within its sphere of influence or the gravitational field. The liquid water on Earth is present within its gravitational field, hence is held strongly in place on the surface of the Earth.
