Explanation:
Gravitational force is given by
[tex]F=\frac{GMm}{r^2}[/tex]
G=6.674×10⁻¹¹ m³⋅kg⁻¹⋅s⁻²
M = Mass of object 1
m = mass of object 2
r = Distance between objects.
Here only variable is r value.
In case 1
[tex]100=\frac{GMm}{(1000+500)^2}\\\\GMm=100\times 1500^2[/tex]
In case 2
[tex]F=\frac{GMm}{(1000+500+500)^2}\\\\F=\frac{GMm}{2000^2}\\\\F=\frac{100\times 1500^2}{2000^2}\\\\F=56.25N[/tex]
Option C is the correct answer.