First, you need to count copper mass in alloy.
Second, you have to make an equation an find x ( the copper mass must be added). The answer is: 13,5g pure copper
Mass C₂H₂ needed : 22.165 g
<h3>Further explanation</h3>
Reaction
2C₂H₂+
5O₂ ⇒ 4CO₂ + 2H₂O
75.0 grams of CO₂ , mol CO₂ (MW=44 g/mol) :

mol C₂H₂ :

mass C₂H₂ (MW=26 g/mol) :

Answer : C) 5.0 x 10^-3 M
if the concentration of Ba(OH)2 is 2.5 x 10^-3 M
then the OH- concentration is twice this so your answer is C.