The chemical name of alum used in sedimentation is—

• a$\mathrm{K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O}$
• b$\mathrm{KCl \cdot AlCl_3}$
• c$\mathrm{Na_2SO_4}$
• d$\mathrm{CaCO_3}$