(A) Civil War
(B) World War I
(C) ** World War II
(D) Revolutionary War
EXPLANATIONS BELOW
Concept note-1: -During World War II, sending and receiving codes without the risk of the enemy deciphering the transmission required hours of encrypting and decrypting the code. The U.S. Marine Corps, in an effort to find quicker and more secure ways to send and receive code, enlisted Navajos as code talkers.
Concept note-2: -Marine Corps leadership selected 29 Navajo men, the Navajo Code Talkers, who created a code based on the complex, unwritten Navajo language. The code primarily used word association by assigning a Navajo word to key phrases and military tactics.
Concept note-3: -Most people have heard of the famous Navajo (or Diné) code talkers who used their traditional language to transmit secret Allied messages in the Pacific theater of combat during World War II.
Concept note-4: -Lloyd Oliver, a Navajo Code Talker, operates a field radio while attached to a Marine artillery regiment in the South Pacific in 1943. Despite the thousands of messages that Code Talkers sent during WWII, their code was never broken by the Japanese or the Germans, who were very good at decryption.