Which state borders only one other U.S. state -- Maine or California?

Maine is the state that borders only one other U.S. state, which is New Hampshire. This unique geographical characteristic is due to Maine's location in the northeastern corner of the country, where it is bordered by the Atlantic Ocean to the southeast and Canada to the north and northwest, limiting its land borders to just New Hampshire.

In contrast, California shares its borders with several states including Oregon, Nevada, and Arizona. Therefore, it does not fit the criterion of bordering only one other state. Texas and Florida also have multiple neighboring states, further distinguishing Maine as the correct choice in this context.