(I) Ring structure of chlorophyll.
(II) Helps to  maintain ribosome structure.
(III) Activator of RuBisCO and PEPcase.
     The above roles are played by

• A

  ${\mathrm{Mg}}^{2+}$

• B

  Cl

• C

  ${\mathrm{Mn}}^{2+}$

• D

  ${\mathrm{Ca}}^{2+}$