Looks like we can choose any d we want, so why not 1m? a = g*m/d^2 = (6.673*10⁻¹¹)(5.98*10²⁴)/(1²) = 3.97*10⁻¹⁴ m/s². This would occur within the earth and would certainly be non-physical if we considered general relativity, but it works. Otherwise, just choose a different d, why not 1.50*10¹¹ m, the mean distance from the earth to the sun.