Which DO in northwest Spain is exclusively red and produced from the Mencia grape?

Bierzo DO is indeed the correct answer because it is a Denominación de Origen located in the northwest of Spain that focuses exclusively on red wines made primarily from the Mencia grape. This region is known for its unique terroir, which allows Mencia to flourish, producing wines that are aromatic with good acidity, offering flavors of dark fruits, floral notes, and sometimes herbal or mineral characteristics.

The other options listed pertain to different styles of wine or grape varieties. Rias Baixas DO is primarily known for its white wines made from the Albariño grape, focusing on freshness and minerality. Rueda DO is another white wine region, mainly producing wines from the Verdejo grape, renowned for its crisp and zesty profile. Valencia DO, while producing a range of wines, is also not exclusively red and features several grape varieties, including both red and white. Thus, referring to Bierzo's dedication to Mencia makes it clear why it stands out as the exclusive red wine DO in the area.