Final answer:
Clicking the yellow (middle) button on an iMac application window minimizes the window, making it an icon in the dock. It neither enlarges the window, turns it into a small square, nor quits the application.
Step-by-step explanation:
When a user clicks the yellow (middle) button at the upper left corner of an application window visible in the iMac interface, the window gets minimized. This specific action is commonly used in macOS, Apple's operating system for their range of computers, including the iMac. The effect of clicking the yellow button is that the window shrinks to become an icon in the dock. It neither makes the window larger nor shrinks it to a small square in the main menu bar. Additionally, clicking the yellow button does not cause the application to quit; it simply minimizes the window, allowing users to access other windows or applications while keeping the minimized application running and quickly accessible from the dock.