First, balance the reaction as follows
2 C2H2 + 5 O2 = 4 CO2 + 2 H2O
Then, there are 2 moles of water produced every time that 5 moles of oxygen react with 2 moles of acetylene.
To find how many moles of water (X moles water) will be formed when 3.8 moles of oxygen are used up in the reaction,
X moles water = [2 moles H2O/ 5 moles O2] * 3.8 moles O2
X moles water = 1.52 moles