how to align vertices on z-axis

it is a bit of time I do not use blender and I do not remember how to align vertices on z axis…like in the attachments.

obviously I do not want to change the thickness of the tube.I want to slide the vertices so that they are all the same height z axis. like a edge slide …

so I do not mean z-s-zero



I’d say delete what you have there and use keys K,Z,C, slide mouse (keeping at Y axis direction) over the object, left click, Enter if you want that blue line loopcut.
In other words - use Knife tool cut through while snapped to axis function.

it is a valid solution