Final answer:
The underlined portion of sentence 1 should be kept because it defines a key concept in the passage, distinguishing the small size of tiny houses in comparison to the average American home.
Step-by-step explanation:
The underlined portion of sentence 1 should be kept because it provides information that helps define a key concept in the passage. The information about the size of tiny houses compared to the average new home in the United States is crucial to understanding the concept of the tiny house movement. This comparison establishes the significantly smaller size of tiny houses and how they efficiently use their limited space, which is central to the movement's advocacy.
Deleting this portion would remove the context necessary for a reader to grasp the extent of what living in a tiny house entails, especially when contrasted with the average American home. Consequently, option A is the best choice: Keep it because it provides information that helps define a key concept in the passage. Details regarding the size of the homes directly support the purpose of the tiny house movement, making the underlined section essential rather than superfluous.