How a list of image source options can be specified to achieve image responsiveness, in terms of the image resolution (using x-descriptors)?

By using the srcset attribute inside the img element, and specifying the x-descriptors (1x, 2x, etc.):

<img src="/files/16797/clock-demo-200px.png"
       srcset="/files/16864/clock-demo-400px.png 2x, /files/16797/clock-demo-600px.png 3x">

Note: This method does not take into account the screen size of the viewport size.