Final answer:
Dorothea Dix was the performer who advocated for better treatment of individuals with mental illnesses, leading to foundational changes in the care and perception of mental health.
Step-by-step explanation:
The performer who tried to ensure better treatment for people with mental illnesses was b) Dorothea Dix. Dorothea Lynde Dix was a pivotal figure in the movement for mental health reform in the 19th century. Her campaign began after witnessing the deplorable conditions of mentally ill individuals housed within a jail in Cambridge, Massachusetts. Dix's efforts included an extensive study of similar institutions and advocacy for the establishment of state mental asylums. Throughout her lifetime, Dix's relentless dedication resulted in the foundation of thirty-two mental hospitals, the improvement of penitentiary conditions, and progress in the public's perception of mental health illnesses.