To achieve a responsive layout in Salesforce Aura components, the lightning:layout and lightning:layoutItem components utilize a 12-column grid system. The size attribute defines the default behavior, which targets the smallest devices (mobile). In the original code, size="12" is applied to all three items. Since each item occupies the full 12 columns, they stack vertically in three rows.

To meet Requirement 2 (displaying in a single row for tablets and desktops), the three items must share the 12-column horizontal space. Mathematically, $12 \div 3 = 4$. Therefore, each item must be assigned a size of 4 for larger screen breakpoints.

Option D is the correct implementation. It maintains size="12" for mobile (Requirement 1) and sets mediumDeviceSize (tablets) and largeDeviceSize (desktops) to 4. This ensures that on any screen larger than a phone, the three items will sit side-by-side in a single row.

Option A is incorrect because mediumDeviceSize="6" would only allow two items per row ($6 + 6 = 12$), forcing the third item to a second row. Option C is incomplete as it only specifies the largeDeviceSize, potentially leaving tablets in a stacked configuration. Option D follows the best practice of explicitly defining the span for all relevant device categories to ensure a consistent user experience.