The sample with greatest number of moles of Cl is calculated as follows
calculate the number of moles of Cl in each sample by multipying mole with number of chlorine molecule
Hcl= 3.8 moles x 1 = 3.8 moles
NaCIO3 = 4.2 moles x 1 = 4.2 moles
CH2Cl2= 1.7 moles x2 = 3.4 moles
Mg (ClO4)2 =2.2 moles x 2 = 4.4 moles
Mg(Cl)4)2 sample is the one with greatest number of Cl moles