Which grape is the only red grape of the North and a major blending grape in the South?

Syrah is recognized as the only red grape that is extensively grown in the Northern Rhône Valley and is also a significant blending grape in the Southern Rhône and throughout the south of France. In the north, it is celebrated for producing some of the world's finest and most structured red wines, particularly in regions such as Côte-Rôtie and Hermitage. Here, the grape thrives due to the cooler climate, which helps preserve its acidity and aromatic complexity.

In the south of France, particularly in the Rhône Valley's warmer regions, Syrah often plays a complementary role in blends, providing depth, color, and tannin to wines that include varieties like Grenache and Mourvèdre. Its ability to adapt to different climates while contributing distinctive characteristics makes it a cornerstone of both northern and southern wine productions.

The other options, while significant in their own right, do not fulfill the criteria set by the question. Grenache is the dominant grape varietal in the Southern Rhône rather than a northern one. Viognier is primarily known as a white grape and does not fit the red varietal requirement. Merlot, while a popular grape, primarily hails from Bordeaux and is not associated with the Rhône Valley as a blending grape in the same context as Syrah