Which factor allows earth to hold onto liquid water?
a. strong gravity
b. weak salinity
c. freezing temperatures
d. presence of living things?

The answer is a. strong gravity.

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.

where is the scientific experiment?

c. earths continental crust is thicker than its lithosphere.

because, the lithosphere is the outermost layer of the earth/ 100 km thick and is defined by its machanical properties.the lithosphere contains oceanic and continental crust that varies in age and thickness across locations and geologic time. the lithosphere is the coolest layer of the earth in terms of temperature. the term lithosphere should not be confused with the use of "geosphere" which is used to indicate all of earth systems, including the atmosphere, hydrosphere   and biosphere. the surface of the earth can be divided into the continental crust and the oceanic crust.   the continental crust is made mostly of granite while the oceanic crust is made of basalt. the average thickness of the continental crust is 25 miles and the average thickness of the oceanic crust is 5 miles.   three types of rock, igneous, sedimentary, and metamorphic, make up the surface of the earth. that's why 3 option is preferable.