Final answer:
The 4-letter word indicating dry champagne is 'brut', with 'extra brut' signifying extremely or totally dry champagne.
Step-by-step explanation:
The 4-letter word that indicates a champagne is dry is brut. When a champagne is described as "extra" brut, it means that the champagne is extremely or totally dry. This term is used to classify the sweetness level of champagne, which is determined by the amount of residual sugar in the wine after the fermentation process is complete. The less residual sugar, the drier the champagne. The scale moves from sweet to dry in the following order: Doux, Demi-Sec, Sec, Extra Sec, Brut, and Extra Brut. Therefore, the correct answer is: a. brut.