If the number of moles end inside a container were tripled while keeping the pressure P constant, what will happen to the volume V? All right, so in this question, they're telling us that our pressure is being held constant. So just ignore it. What's changing our moles? And we have to understand what effect will that have on my volume.
Looking at the ideal gas law, PV=NRT or only paying attention to volume and molds the other variables we can remove. By doing that, we see that it simplifies further into V=N. So this means that volume and moles have a direct relationship to each other. They're directly proportional, meaning whatever happens to one, the same thing happens to the other.
Since our moles are being tripled, that would mean that my volume would also have to be tripled. This means that the answer would have to be option C. So again, sometimes it would be faced with questions like this where you're not given actual numbers, but you have to understand the direct or possibly inverse relationship between a pair of variables from the ideal gas law.