Answer
All are correct.
Work Step by Step
As the total mechanical energy is conserved, we have $$E_f=E_0$$ $$KE_f+PE_f=KE_0+PE_0$$ $$KE_f-KE_0=-(PE_f-PE_0)$$
So if kinetic energy decreases, or $KE_f-KE_0\lt0$, then $PE_f-PE_0\gt0$, meaning gravitational potential energy increases, so a) is correct.
If gravitational potential energy decreases, or $PE_f-PE_0\lt0$, then $KE_f-KE_0\gt0$, meaning kinetic energy increases, so b) is correct.
If kinetic energy does not change, or $KE_f-KE_0=0$, then $PE_f-PE_0=0$, meaning gravitational potential energy does not change, too, so c) is correct.
