Which is the best definition for the term "american frontier"? a) a fixed boundary line b) the atlantic or pacific coastline c) the territory gained in the gadsden purchase d) a shifting area where pioneer settlement ended

The correct answer is D) a shifting area where pioneer settlement ended.

The best definition for the term "American frontier" is a shifting area where pioneer settlement ended.

In the 19th century, the concept of American Frontier was the demarcation of the westward lands conquered or conquered by the Pioneers. It can be understood as the time of expansion of white settlers in the United States territory and the fulfilling of the "Manifested Destiny" supported by US President Thomas Jefferson after the acquisition of the Louisiana Territory that belonged to the French.