Think about context, not just screen size. A tablet user on a sofa has different needs from a desktop user at a desk, even if screen sizes are similar. Design for the context of use.
Strong answers go beyond CSS media queries: considering touch vs pointer input, different contexts of use (desk vs commute), content priority shifts between screen sizes, progressive enhancement, and testing on actual devices. Best candidates discuss container queries, fluid typography, and designing for the continuum of screen sizes rather than fixed breakpoints.
Tests modern design thinking. Candidates who only think in terms of "mobile, tablet, desktop" breakpoints are behind. Those who consider input method, context, and fluid layouts demonstrate current practice.