Which grape variety is occasionally used in the south, alongside being the principal grape of Condrieu?

The correct answer is Viognier, which serves not only as the principal grape variety in the Condrieu appellation located in the northern Rhône Valley, but is also occasionally used in blends and other wines in the southern Rhône. This aromatic white grape is known for its floral and fruity characteristics, often presenting aromas of peach, apricot, and honeysuckle, which contribute to the richness and complexity of wines made from it.

In the context of Condrieu, Viognier is exclusively used to produce high-quality white wines, showcasing its unique characteristics. Its limited cultivation and specific terroir requirements make it a distinctive variety in this region. Viognier is sometimes blended with other grape varieties in the southern Rhône to add depth and aromatic complexity, even though it is predominantly celebrated for its individual expression in Condrieu.

The other grape varieties mentioned, such as Syrah, Grenache, and Cabernet Sauvignon, while significant in their respective regions and styles, do not share the same exclusive or principal association with Condrieu as Viognier does. This makes Viognier the clear choice as the correct answer.