The correct answer is A. 1.8 gram of H2O.
1 mole of any substance contains $6.022\times10^{23}$ molecules. The atomic weight of silver is 108, so 1 mole of silver contains $6.022\times10^{23}$ atoms of silver. 21.6 grams of silver is equal to 2 moles of silver, so it contains $2\times6.022\times10^{23}$ atoms of silver. 1.8 grams of water is equal to 0.5 moles of water, so it contains $0.5\times6.022\times10^{23}$ molecules of water. Therefore, the number of atoms present in 21.6 gram of silver is the same as the number of molecules in 1.8 gram of water.
The number of molecules in 12 moles of KMnO4 is $12\times6.022\times10^{23}$ molecules. The number of molecules in 0.6N H2SO4 is $0.6\times6.022\times10^{23}$ molecules. The number of molecules in 4.6 gram of C2H5OH is $4.6\times6.022\times10^{23}$ molecules. Therefore, the number of atoms present in 21.6 gram of silver is not the same as the number of molecules in any of the other options.