Answer: D) all the other planets, each contributing according to its.
Explanation: When we have a plural subject, followed by the word "each" the verb should be in singular form, because it corresponds to "each" and not to the plural subject. In this case, we have "each" referring to the Sun an all the planets, but the verb and possessives must be singular so it's "its mass" and not "their mass", that leaves us just with options C and D, the correct answer is D because the structure of option C "each of which contributing" is incorrect, it should be "each of which contributes" or "each contributing" (like option D), so the complete correct sentence would be: In his experiments with gravity, Isaac Newton showed how the motion of each planet in the solar system results from the combined gravitational pull of the Sun and all the other planets, each contributing according to its mass and distance from the others.