What benefit does the size of the Earth serve? 

A)  Its size keeps it in orbit at a constant distance from the sun. 
B)  Its size provides enough gravitational mass to keep the light gaseous molecules in its atmosphere from flying off into space. 
C)  Its size keeps the iron-nickel core solid. 
D)  Its size prevents transfer of geothermal energy to the surface of the planet.

The Earth's size provides enough gravitational force to keep the lighter gaseous molecules in its atmosphere from escaping into space. This allows for the retention of the atmosphere and the protection it provides to life on Earth. A and C may be true to some extent, but they are not the primary benefits of Earth's size, and D is incorrect as geothermal energy is transferred to the surface of the planet.