The uploaded UX Designer study guide identifies A. Specifying breakpoint sizes as the responsive design best practice.
Responsive design must account for changes in available screen width. Breakpoints provide defined thresholds at which the layout can adapt so that content remains usable and appropriately structured across different screen sizes.
The study guide therefore selects specifying breakpoint sizes as the practice that directly supports responsive behavior.
Option B, using interaction models, is not the responsive-layout mechanism described in this question. Interaction models can define how users work with an interface, but they do not themselves establish when the visual layout should change in response to screen dimensions.
Option C, creating a separate mobile-specific site, is not the best practice identified by the guide. The responsive approach is intended to allow one experience to adapt according to available space rather than requiring an entirely separate site merely because the device is smaller.
The important distinction is between adaptive layout behavior and maintaining separate experiences. Breakpoints allow the design to respond systematically as the viewport changes.
For the UX Designer, the practical objective is to define when content should resize, reorganize, wrap, or otherwise change presentation so that the experience remains usable across device dimensions.
Study Guide reference/topics: Responsive Design, breakpoints, screen sizes, adaptive layouts, UX Fundamentals.
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