The decline of normal schools around the turn of the 20th century was primarily due to:
b. They were replaced by colleges that offered a degree in education.
Normal schools were established to train teachers and were often two-year institutions. However, as the demand for more qualified teachers grew, many states began to require advanced degrees for teaching positions. This led to the development and expansion of colleges and universities that offered bachelor's degrees in education, which ultimately replaced normal schools as the primary means of teacher training.