You will realise that the number of hydrogen atoms do not add up to the original 12.
There are 4 hydrogen atoms in each molecule of pyruvic acid. Since there are two molecules, total number of hydrogen = 4 x 2 = 8
So 4 atoms of hydrogen are lost. When hydrogen is lost, the molecule becomes oxidised. Since one big molecule is broken into two smaller ones, decomposition occurs.
So the answer is A: oxidation and decomposition.