What causes the mobile layout to display when using a mobile-optimized template?
A. The screen size of the device that is used to view the email B. The specific device that is used to view the email C. The email client detection tracking pixel D. A subscriber attribute that dictates a mobile layout preference
According to Salesforce: "A responsive layout uses a piece of CSS3 code called a media query to detect the pixel size of the screen on which the email is opened. Based on the screen size detected, multiple stylesheets may be triggered to alter the layout and best respond to a particular screen size."
The screen size of the device that is used to view the email is the anwser.
Go to : salesforce .com/products/marketing-cloud/resources/email-design-toolkit/
As you can see, under the design tab: "CSS3 media queries are added to the email to detect screen size. Then, styles are overwritten to adjust text size, frame width, and image size for a more usable experience."