First, look at the equation:
Fgravity= (G*m1*m2)/(r^2)
If you are looking for a doubling of the attractive force, you could change the mass of the earth, the mass of the sun, or the distance between the two.
Since you have "which of the following" in your question, it is difficult to know what your exact options are, so here are a few ideas:
Doubling the earth or the sun mass will double the force due to the direct relationship. I like to think of equations like this:
m1 goes up = F going up
m2 goes up = F going up
They directly correspond to each other algebratically so if you double the mass, you will also double the force.
However, the distance between the two, the "r" value, is more tricky since it is in the quotient and squared. Look at the case if we were to decrease the distance of r original to r/sqrt(2)
Fgrav=G m1 m2/r orig. ^2 = G m1 m2/(r/sqrt(2))^2 = G m1 m2 / (r^2/ 2)
Simplifying, you get:
Fgrav = G m1 m2 / (r^2/ 2) = 2*G m1 m2/r^2
Thus, by DECREASING the distance between the earth and the sun by a factor of 1/sqrt(2), you double the force.
Hope this helps! Let me know if I can be of further assistance!
