Correct option (c) Na2SO4 . Al2( SO4)3 . 24H2O
Explanation:
The term alum is used for all the double sulphates having the composition,M2SO4. M'2( SO4)3.24H2O Where M stands for monovalent basic **** such Na+, K+ , etc. and M' for trivalent basic **** such as Al3+,. Cr3+, etc.