Q.Which CSS feature applies a set of style rules only when the screen matches a condition such as a maximum width, making it central to responsive design?
(A) media query
(B) selector
(C) box model
(D) pseudo-class

This tests the main tool of responsive design — the feature that changes styles at different screen widths. …

Responsive design serves different layouts to different screen sizes.

(A) A media query (@media (max-width: 600px) { ... }) applies its rules only when the condition about the screen is true — this is exactly how layouts change at a breakpoint — correct.

(B) A selector chooses which elements a rule targets, but does not depend on screen size — incorrect. …

Forgetting the viewport meta tag. Media queries only work responsively on phones if the page also includes <meta name="viewp …