how is it an SHM

k*qx/(a^2+x^2)^3/2

so force on a particle of say -Q is given by

-k*qQx(a^2+x^2)^3/2

now if x<<a then force = -kqQx/a^3 which is an SHM

so c) is true

but for x------>inf force becomes inverse square so it is not SHM

so only c) is true