Media queries support responsive web design and allow different CSS rules to be applied at different screen sizes, device characteristics, or breakpoints. MDN describes media queries as a CSS mechanism for applying styles depending on media type and device or viewport characteristics, including width, height, resolution, and orientation. This is the foundation of responsive design because a page can adapt its layout for phones, tablets, laptops, and desktop displays without changing the HTML document. Breakpoints are the points where the design changes; MDN identifies them as the points where a media query is introduced to improve layout behavior across viewport sizes. Statement A is not generally correct because media queries are primarily a styling and layout mechanism, not a direct performance optimization. They can sometimes help avoid unsuitable layouts, but performance improvement is not their defining purpose. Statement D is incorrect because media queries are CSS rules; they do not execute or dynamically process JavaScript code. References/topics: responsive design, CSS media queries, viewport width, breakpoints, adaptive layout rules.

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