Carbon dioxide
Oxygen
Nitrogen
Hydrogen
Answer is Wrong!
Answer is Right!
The correct answer is A. Carbon dioxide.
Carbon dioxide dissolves in water to form carbonic acid, which is a weak acid. The reaction is as follows:
$CO_2 + H_2O \rightleftharpoons H_2CO_3$
Carbonic acid can then dissociate into hydrogen ions and bicarbonate ions:
$H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$
The hydrogen ions are what make the solution acidic.
Oxygen, nitrogen, and hydrogen do not dissolve in water to form acids.