Answer
$(a)$ separated by finite distance
Work Step by Step
Mass of the system depends on energy , according to the special theory of relativity like :
$E=mc^2$ . The only energy , in our case , that change with distance like
Potential energy between the charges$=$ $Kq/r$$\times$$q$
$c$ is speed of light that is constant.
Hence the total mass of the system greater when the two charges are separated by finite a distance.
