Answer
Diagram "C" shows the final volume of the balloon for that situation.
Work Step by Step
According to the Avogadro's law, if the pressure and the temperature are constant, the volume is directly related to the number of gas moles.
Therefore, if we add 2.0 moles of $O_2$ gas to the balloon, the number of gas moles will increase, and the volume will also increase. Diagram "C" is the best representation for that.