okay so, the molar mass (Mr) of O2 is 32g
moles = mass/Mr
moleS= 32.4/32= 1.0125mol
the Mr of KCl is 39.1+35.5= 74.6g
the ratio of moles of KCl to O2 is 2 : 3
therefore moles of KCl is (1.0125/3)×2= 0.675mols
mass =moles×Mr=0.675×74.6= 50.355g
rounds up to 50.3g therefore last answer is correct