Answer: Nueva California
Explanation: In 1821, Arizona was on the frontier of the Mexican state of Nueva California, which had a population of more than 100,000. Nueva California was a territory in northern Mexico that included parts of present-day California, Nevada, and Arizona. It was established by the Spanish colonial government in the 18th century as a province of New Spain, and was one of the most populous regions in Mexico at the time. However, the population of Nueva California was largely concentrated in the coastal areas, and the interior regions such as Arizona were relatively sparsely populated. Therefore, while the population of Nueva California as a whole was more than 100,000, the population of Arizona specifically was likely much smaller.