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.
Hope this helped
