Answer
Marie-Thérèse became Dauphine of France after her uncle, King Louis XVIII, acceded to the throne in 1824. The Dauphine is the wife of the Dauphin, who is the heir to the French throne. Louis XVIII had no children of his own, so his nephew, Louis-Antoine, Duc d'Angoulême, became Dauphin. Louis-Antoine married Marie-Thérèse in 1820, and she became Dauphine.