B and D have the same number of protons, so they are the same element. The number of protons in B is less than the number of electrons, so B stands for cation. The number of protons in C is less than the number of electrons, so C stands for anion. The number of protons in A and C is equal to the number of electrons, so they are atoms.

Therefore, choose a and d; b； c； b、d。