Which situation would cause the following equilibrium reaction to decrease the formation of products 2so2(g) + o2(g) forward backward arrow 2so43(g) + energy

The reaction is as Follow,

                      2 SO₂  +  O₂    ⇄    2 SO₃    ΔH = -94.58 kJ mol⁻¹

Changing Concentration:
                                           According to Le Chatelier's principle, the removal of SO₂ and O₂ (or decrease in their concentration) at equilibrium will result in decrease in the formation of SO₃.

Changing Pressure:
                                  As the reaction proceeds by the decrease in volume, so by decreasing the pressure will shift the equilibrium in backward direction decreasing the concentration of product.

Changing Pressure:
                               Since the forward reaction is exothermic, the equilibrium will be shifted in backward direction by increasing the temperature.

Ido not understand. 

Burned piece of steel wool does not gain mass. so only d could be true.