One because Potassium is in the first column on the periodic table. Any element in the first column on the periodic table has one valence electron in their outer shell.
C A has 2H 2O on the left and 4H 2O on the right B has 4H 4O on the left and 4H 2O on the right D has 2H 2O on the left and 2H 4O on the right C has 2H 2O and 2H 2O so is balanced.
