To create a responsive design in Salesforce Aura components, the lightning:layout and lightning:layoutItem components utilize a 12-column grid system based on the Salesforce Lightning Design System (SLDS). The issue in the original code is that size="6" is hardcoded, which forces each item to occupy 50% of the container width regardless of the screen size. On small mobile screens, 50% width is often insufficient for text content, leading to horizontal scrolling or overlapping.

Option B resolves this by using device-specific size attributes. By setting smallDeviceSize="12", each item is instructed to take up the full width (12 out of 12 columns) on mobile devices. This causes the items to stack vertically instead of sitting side-by-side. The mediumDeviceSize="6" and largeDeviceSize="6" attributes ensure that on tablets and desktop screens, the items return to a side-by-side layout (50% width each).

For this stacking to occur, the parent lightning:layout must have the multipleRows="true" attribute enabled. This allows the layout to wrap items to a new line once the total column count in a single row exceeds 12. Without multipleRows="true", the items would try to squeeze into a single line regardless of their individual size settings, which would not solve the horizontal scrolling problem. Option B is the only choice that correctly applies the grid logic to handle multiple breakpoints effectively.