Answer is: H2SO4.
Balanced reaction of decomposition of sulfuric acid:
2H₂SO₄ → 2H₂O + 2SO₂ + O₂ at 830°C.
Sulfuric acid is strong acid.
Decomposition is reaction where one substance (in this example sulfuric acid) is broken down into two or more simpler substances (in this example water, sulfur dioxide and oxygen).
