Which of the following is a key producer in Montalcino?

In Montalcino, a prominent wine region in Tuscany known especially for the production of Brunello di Montalcino, Castello Banfi is recognized as a key producer. This estate, founded in the 1970s, has made significant contributions to the stature and recognition of Brunello di Montalcino both domestically and internationally. The winery is known for its commitment to quality and innovation, and has been influential in advancing modern wine-making techniques in the region.

The other options mentioned pertain to different regions or wine styles: Marchesi Antinori is renowned for its estate in Chianti Classico and other regions, Felsina is also a well-known producer in Chianti, and Terresole does not hold the same recognition in the context of Montalcino. Therefore, the answer reflects Castello Banfi's established role and prominence in the Montalcino wine community.