Your answer would be the last option: What will happen to the individual sections cannot be determined, but the size of the two sections together increases.
This is because angles on a straight line add to 180°, so if angle C decreases, angles A and B must collectively increase to make up for the loss. For example, if C = 20° and it decreases to 15°, A + B + C would equal 175° and so A and B would need to increase to make it 180°.
The reason we can't determine what happens to the individual sections is that they could either both increase, or one could increase and one could decrease, as long as they collectively make up for the decreased angle.
I hope this helps! Let me know if you have any questions :)
