A.
the left side - 2 atoms of hydrogen, 1 atom of oxygen
the right side - 2 atoms of hydrogen, 4 atoms of oxygen
not balanced
B.
the left side - 4 atoms of hydrogen, 2 atoms of oxygen
the right side - 4 atoms of hydrogen, 2 atoms of oxygen
balanced
C.
the left side - 4 atoms of hydrogen, 2 atoms of oxygen
the right side - 2 atoms of hydrogen, 4 atoms of oxygen
not balanced
D.
the left side - 2 atoms of hydrogen, 1 atom of oxygen
the right side - 4 atoms of hydrogen, 4 atoms of oxygen
not balanced
The answer is B.