ANS:D - 2.5

To determine the theoretical volume of oxygen required for the complete combustion of 1 Nm^3 (cubic meter) of acetylene in oxy-acetylene welding, we need to refer to the stoichiometric equation for the combustion reaction: 2C2H2 (acetylene) + 5O2 (oxygen) → 4CO2 (carbon dioxide) + 2H2O (water) From the balanced equation, we can see that the stoichiometric ratio of acetylene to oxygen is 2:5. So, for complete combustion of 1 Nm^3 of acetylene, we would need: (5/2) × 1 Nm^3 = 2.5 Nm^3 of oxygen. Therefore, the theoretical volume of oxygen required for the complete combustion of 1 Nm^3 of acetylene in oxy-acetylene welding is 2.5 Nm^3. Among the options provided, the correct answer is:

• 2.5 Nm^3