According to European law, what botanical must be included in Vermouth?

Vermouth must include botanicals from the wormwood family, specifically Artemisia absinthium, which is commonly known as bitter wormwood. This is a requirement under European law that ensures the distinctive flavor profile characteristic of vermouth. The inclusion of wormwood is essential because it not only defines the spirit but also contributes to its bitter taste.

Other botanicals can be added for flavor complexity, but the presence of wormwood is what legally classifies the product as vermouth. This is why the answer indicating plants from the wormwood family is correct, as it aligns with the legal definition and traditional production methods of vermouth in Europe.