The correct response that replaces the parentheses portion of the phrase "Nous sommes allées (à des concerts)" is C) Vous êtes allées à laquelle ?
This question is asking about the feminine singular form because "allées" is the past participle of the verb "aller" and agrees with the feminine plural "nous." The correct form for "à des concerts" is "à laquelle" because "concerts" is plural and feminine.