The Balmer series refers to a series of spectral emission lines of the hydrogen atom arising from electronic transitions from any higher level and terminating at the the energy level n= 2.
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.