26) C

Volume of sulphuric acid = 12.5 = 0.0125 dm³
Concentration of sulphuric acid = 2 mol/dm³
Moles of sulphuric acid = concentration x volume = 0.0125 x 2 = 0.025 mol

Molar ratio (XOH: H₂SO₄) = 2:1
Moles of XOH = 0.025 x 2 = 0.05 mol
Mass of XOH = 2.8g ≈ 3

Mr of XOH = mass ÷ moles = 3 ÷ 0.05 = 60
Ar of H = 1
Ar of O = 16
Ar of X = 60 – 1 – 16 = 43

This is closest to 39. So C is the answer.