(D) 1 : 8
The balanced equation is 2H₂O → 2H₂ + O₂. Mass of H₂ = 4 g, mass of O₂ = 32 g, so M_H : M_O = 4 : 32 = 1 : 8.
The question asks for the mass ratio, not the mole or volume ratio. From the balanced equation, 2 moles of H₂O give 2 moles of H₂ (mass = 4 g) and 1 mole of O₂ (mass = 32 g). So the mass ratio is 1 : 8. A common mistake is to pick 1 : 2 (mole ratio) instead of the mass ratio.