+1 just what I’d been thinking!
Liquid and responsive are basically the same for a buyer. Plus, if you set it to liquid and your buyer resizes , he can still see the fact that it’s liquid/responsive!
They should actually be separate options. Responsive themes are set to work differently at “X and below” resolutions. A liquid theme would not have a lot of white space of a 2560×1800 screen but a responsive theme would.