Which of the following is not classified as a Grand Cru?

The answer provided is supported by the classification system used in Burgundy wines. "Les Combettes" is not a recognized Grand Cru vineyard, whereas the other options represent established Grand Cru sites.

Chateau Margaux is classified as a first growth in Bordeaux, which is a prestigious category that indicates exceptional quality. While its classification is different from that of Burgundy's Grand Cru, it still embodies the elite status associated with top-tier vineyards.

La Tache is indeed a Grand Cru vineyard, and it belongs to the esteemed Domaine de la Romanée-Conti, which is renowned for producing some of the finest Pinot Noir in Burgundy.

Chablis Grand Cru refers to a specific classification in the Chablis region of Burgundy, known for its high-quality Chardonnay wines, and is universally acknowledged as a Grand Cru.

Understanding these classifications helps grasp the hierarchy and quality standards prevalent in the French wine regions, emphasizing that while all mentioned vineyards carry the weight of prestigious classifications, "Les Combettes" does not hold the designation of Grand Cru.