6) D

Be careful! The question states that 12g of CARBON (i.e. not carbon dioxide) is used in stage 2.

In the 2nd equation:
Mass of carbon = 12g
Mr of carbon = 12
Moles of carbon used = 12/12 = 1 mol
Molar ratio of C and 2CO = 1 : 2
Hence mole of CO produced = mole of carbon (i.e. 1 mol) x 2 = 2 mol

In the 3rd equation:
It is stated that all of the CO formed in the second equation is used in the third equation.
Moles of CO used = 2 mol
Molar ratio of CO used and CO₂ formed = 3:3 = 1:1
Moles of CO₂ formed = 2 mol
n of CO₂ = 2

Mr of CO₂ = 12 + 16 + 16 = 44

mass = n x Mr = 2 x 44 = 88g