We know that this is a combustion question since it contains a hydrocarbon being burned with oxygen. Therefore the products are water and carbon dioxide.
C₂H₂+O₂⇒H₂O+CO₂
Balancing this,
2C₂H₂+5O₂⇒2H₂O+4CO₂
We know that 2 moles of C₂H₂ will react with 5 moles of O₂. From the balanced equation.
We can set up our ratios:2:5=35:x
2x=175
x=87.5
Therefore it would require 87.5 moles of O₂