Final answer:
In the United States, there are three main social classes: upper, middle, and lower class. The missing word to complete the sentence is 'Poor'.
Step-by-step explanation:
In the context of social class in the United States, there are three main classes: the upper class, the middle class, and the lower class. The upper class consists of the very rich, who make up only a small portion of the population but own a significant share of the country's wealth.
The middle class is the largest class and includes the majority of the population. The lower class is the class with the least wealth and resources. So, the missing word to complete the sentence is Poor.