Which of these American cities has fewer than 1,000,000 people?

The answer is San Francisco, California, primarily because it has a smaller population compared to the other options listed. While all the cities mentioned are significant urban centers in the United States, San Francisco's population falls below the one million mark. With an estimated population of around 883,305 as of 2020, San Francisco is known for its iconic landmarks, cultural diversity, and thriving tech industry. Its relatively lower population size, compared to cities like Chicago, Houston, and Philadelphia, may be attributed to factors such as geographical constraints, limited land availability, and high cost of living.