To answer this question, we only have to look for the molar masses of each of the compounds and compare which one of them is the greatest, this is because the molar mass of a compound is the mass of one mole of that compound.
The molar mass of NI3 is 394.72g/mol.
The molar mass of H3PO4 is 97.99g/mol.
The molar mass of KCl is 74.55g/mol.
The molar mass of FeCl3 is 162.2g/mol.
It means that 1.0mole of NI3 is the one with the greatest mass. The correct answer is A.