What is the union of the flag?

The union of the flag refers specifically to the blue field where the stars are displayed. This blue section, also known as the canton, represents the unions of the states within the country, signified by the stars. Each star corresponds to a state, and the blue field symbolizes the unity of these states. It visually signifies the collective identity and governance established through the union, making it a crucial element of the flag's symbolism.

In the case of the choices given, only the blue field properly defines the union, as it encompasses both the color and the arrangement of the stars that symbolize the states' alliance and the national integrity. The red stripes, star formation, and white background each have their own meanings and significance within the flag but do not represent the term "union" as accurately as the blue field does.