Final answer:
Historians often consider Antonie Brentano as the likely 'Immortal Beloved' to whom Beethoven addressed his heartfelt love letter.
Step-by-step explanation:
Historians believe that Beethoven's "Immortal Beloved" refers to an unidentified woman with whom the composer fell deeply in love and to whom he wrote a famous love letter in 1812. The mystery of her identity has intrigued scholars and music enthusiasts alike, with candidates including Antonie Brentano, Josephine de Brunswick, and Amalie Sebald. However, Antonie Brentano is often considered the most likely option, based on evidence from Beethoven's letters and the circumstances of his life at that time.