Final answer:
The correct answer is option a) Nevada.
Step-by-step explanation:
The US military installation commonly known as Area 51 is situated in the state of Nevada. This location is well-known for its secrecy and as the center of various UFO-related conspiracy theories. Its connection to the military's nuclear testing history is evident, as US soldiers have observed test explosions of nuclear devices in this area, particularly during the 1950s.
Relating to another piece of historical context, as a result of the Adams-Onís Treaty, the United States gained territory from Spain; the answer to this piece of history is A. Florida. Other options such as New Mexico, California, and Nevada were not part of this specific treaty acquisition.