A UX Designer is asked to design a responsive page.

The SLDS utility that provides the most flexible system to meet the requirements of designing a responsive page is Grid. Grid is a utility that allows you to create a layout using a series of rows and columns that house your content. Grid uses a 12-column structure that can be adjusted based on breakpoints, which are predefined screen sizes that trigger a change in the layout. Grid also supports nesting, alignment, ordering, and offsetting of the columns. By using Grid, you can create a responsive page that can adapt to different screen resolutions and devices12 References: [Grid - Lightning Design System], [Mastering Salesforce Lightning Design System Grids and Lightning Layouts]

https://www.lightningdesignsystem.com/utilities/grid/

https://developer.salesforce.com/blogs/developer-relations/2017/04/mastering-salesforce-lightning-design-system-grids-lightning-layouts