Answer:
65.712 grams of oxygen has reacted.
Step-by-step explanation:

Mass of acetylene = 21.39 g
Moles of acetylene =

According to reaction , 2 moles of acetylene reacts with 5 moles of oxygen gas.
Then 0.8214 moles of oxygen gas will react with :
of oxygen gas.
Mass of 2.0535 moles of oxygen gas :
2.0535 mol × 32 g/mol = 65.712 g
65.712 grams of oxygen has reacted.