Assuming that the beer flows out at an even pace where when it reaches a tube, it stops filling, we can rule out 2, 4, and 5 due to a higher tube, leaving 1, 3, 6, and 7. Next, 7 is unfillable since its tube is blocked off, and 1 and 3 are a lot higher up than 6 and have tubes connected to the sides rather than on top.
So the correct answer to this question would be glass #6