Which of the following is the southernmost key region of Chile?

The Southern Region of Chile is recognized as the southernmost key wine-producing area in the country. This region, which includes areas such as the Bio-Bio and Itata Valleys, is known for its unique climatic conditions and diverse terroirs that contribute significantly to the character of its wines. The Southern Region is emblematic of cooler climate viticulture, benefiting grape varieties that thrive in such an environment, including those like Pinot Noir and Chardonnay.

In contrast, the other regions mentioned, such as Coquimbo, Aconcagua, and Central Valley, are located further north and thus are not classified as the southernmost regions. These northern regions are better known for their warmer climate and the production of robust red wines, especially in the Coquimbo region and the more renowned Cabernet Sauvignon from the Aconcagua. Therefore, identifying the Southern Region as the correct answer highlights its geographic and viticultural significance within Chile's wine landscape.