#1 Scale Challenge:
When the article talks about the Scale challenge it is referring to the obstacle faced by developers when trying to adapt their website to all screen sizes and inputs. In (Current Year) the number of devices that can access the web are countless. Even some fridges can load YouTube now a days. That being said, developing a different version of your website for each individual device is an impossible task. The scale challenge is referring to this impossible task and how to overcome it.
#2 Flexible Design:
Flexible design is not enough to overcome the scale challenge because it falls apart when the resolution goes below a certain threshold. Flexible designs work wonderfully when being viewed on a larger screen then they were designed for but when you bring that resolution down to what you would see on a phone for instance the designs quickly break. Its not enough to have things scale to fit on what screen they are being viewed on. Once you get past a certain point you need to shift elements around and make things responsive.
#3 Media Query:
A Media Query is a property in CSS that allows us to enforce CSS rules only when users device meets certain parameters. It lets us develop one site that can adapt and scale on any size screen. Without media queries the internet would not be what it is today.