>>715791473
The correct answer is a straight line parallel to the bottom of the bottle. I realize that this is unintuitive, but the question if phrased in a way that does not account for the treachery of images.
It's not a bottle, it's a picture of a bottle so if you tilt the picture, of course the waterline will be parallel to the bottom.
This is a classic problem, the people making the study didn't realize this. Because people no longer receive an education in philosophy and the arts at university.