Which key region of the southern Rhône is known for producing only rosé wines?

Tavel is recognized as a key region in the southern Rhône that is exclusively dedicated to the production of rosé wines. This AOC (Appellation d'Origine Contrôlée) is celebrated for its distinctive rosés that are known for their depth of flavor and complexity, usually made from Grenache and other varietals. Tavel's unique terroir, which includes the influence of the Rhône river and the local climate, contributes to its ability to produce high-quality rosé that often stands out with its rich color and structure.

In contrast, Provence is famous for its rosé as well but is not limited solely to this style; it produces red and white wines as long as well. The Languedoc and Roussillon regions are also diverse and produce a wide variety of wines, including significant amounts of red and white wines, in addition to rosé. Therefore, Tavel stands out as the specific region within the southern Rhône known solely for red wines, making it the correct answer.