Liqueur de tirage is the mixture used to start the second fermentation inside the bottle in Champagne production. It’s typically a small amount of wine blended with sugar and yeast. When this mixture is added to the base wine and bottled, the yeast consumes the sugar, producing alcohol and carbon dioxide. The CO2 is trapped in the bottle, creating the bubbles and pressure that make sparkling wine. This secondary fermentation also leaves behind lees, which will be removed later during riddling and disgorging. The sweetness of the final wine is controlled later, after disgorgement, by adding dosage. That’s why the correct description is a cocktail of wine, sugar, and yeast added to base wine to kick off secondary fermentation in the bottle. The other options describe different steps or elements: dosage added after disgorgement, yeast breaking down during ageing, or removing the lees.