Which Northern Rhone Cru allows the use of the white grapes Marsanne and Roussane in their red wines?

The correct answer is St. Joseph, as it is one of the few Northern Rhône Crus that permits the inclusion of white grapes Marsanne and Roussanne in its red wine blends. This characteristic is particularly interesting because it adds complexity and aromatic qualities to the red wines produced in this region. St. Joseph's regulations allow vintners to use up to 15% of these white grape varieties in their red wine production, which can enrich the final product with a unique flavor profile.

Côte-Rôtie, while known for its high-quality reds largely made from Syrah, does allow for a small proportion of Viognier to be blended, but not Marsanne and Roussanne for red wines. Crozes-Hermitage follows similar rules to Hermitage and primarily focuses on Syrah for its red wines, without the allowance of white grapes for red wine production. Hermitage itself, although it does allow white grapes in the production of white wines, does not permit their use in red wine blends. Thus, St. Joseph stands out as the correct option among the Northern Rhône Crus for its allowance of Marsanne and Roussanne in red wines.