Final answer:
By 1890, Miami had a population of fewer than 100,000, as the city was not officially incorporated until 1896 with just over 300 residents.
Step-by-step explanation:
By 1890, the number of people who had settled in Miami was fewer than 100,000. During this period, the United States experienced rapid population growth, primarily in urban areas. The Census Bureau, established in 1902, was responsible for calculating the U.S. population. However, in the late 1800s, Miami was not yet the large urban center it would become in later years. In fact, it wasn't officially incorporated as a city until 1896. At that time, the recorded population was just over 300 residents, a far cry from even the lowest option provided in the question. Therefore, the correct answer to the question is A. fewer than 100,000.