Balancing an equation is essentially making it so that both sides of the arrow have an equal amount of each element, hence are "balanced"
So for A), although there are 2 hydrogens on each side, there are 3 oxygens on the right but only two on the left, so it isn't balanced, hence A) is 'wrong'.
For B, the equation IS balanced because there are 4 irons, 6 oxygens, and 3 carbons on each side.
C is unbalanced; only 1 sulfur on left vs 4 on the right, 4 hydrogen on left but 8 on right, and 6 oxygen on left but 16 on right.
D is also unbalanced, as there are 2 magnesiums on the left but 1 on right, and there is one of hydrogen and chlorine in the left but two of each in the right.
So the answer is that A) is the only balanced chemical equation.
