The max.no. of molecules is present in

1)15L of water at STP

215L of H2O gas at STP

3)15gm of ice

4)same in all

The given three options are as follows:
At STP, 15 L of water; 15 L of H2O ; and 15 g of Ice.
If we observed, in three options H2O is present in three different states.
At standard pressure and temperature, the liquid state has maximum density hence it has high number of atoms. The density of liquid state of any compound is equal to the density of gas and solid forms at STP.
Therefore, 15 L of water will have maximum number of molecules then compared to 15 gm of ice and 15 L of H2O gas.