Which of the following is a requirement for "garrafeira" wines in Portugal?

The requirement for "garrafeira" wines in Portugal is that they must have a Denominação de Origem Controlada (DOC) designation. This designation is essential as it indicates that the wine comes from a specific region and meets particular quality standards set forth by the Portuguese wine authorities. The DOC ensures adherence to specific regulatory practices regarding grape varieties, winemaking techniques, and aging processes, thereby supporting the authenticity and quality of the wine being produced.

In the context of the other choices, the production methods or aging requirements do not define a "garrafeira" wine's eligibility. For instance, while aging in barrels is common among high-quality wines, it is not a strict criterion for all DOC wines. Similarly, the requirement for aging 30 months in barrels is specific to certain styles but not universally applicable to all "garrafeira" wines, nor is the constraint on using stainless steel a requirement. The concept of non-vintage specifically refers to wines made from grapes harvested in different years, which also doesn’t pertain to "garrafeira" classifications. Thus, the DOC designation stands out as the core requisite marking these wines.