How did the Navajo Code Talkers (Native Americans fighting in the Pacific theater)
contribute to the war effort?
The Navajo Cole Talkers were able to send secret
messages on the battlefield using the Navajo
language
The Navajo Code Talkers were instrumental in the
development of the atomic bomb
The Navajo Code Talkers were in the first wave of
soldiers when the U. S. landed on the island of Iwo
Jima and Okinawa

The Navajo Cole Talkers were able to send secret messages on the battlefield using the Navajo language.

Explanation:

The Marine Corps recruited Navajo Code Talkers in 1941 and 1942. Philip Johnston was a WWI veteran who had heard about the successes of the Choctaw telephone squad. Johnston, although not Indian, had grown up on the Navajo reservation. In 1942, he suggested to the Marine Corps that Navajos and other tribes could be very helpful in maintaining communications secrecy. After viewing a demonstration of messages sent in the Navajo language, the Marine Corps was so impressed that they recruited 29 Navajos in two weeks to develop a code within their language. After the Navajo code was developed, the Marine Corps established a Code Talking school.

There is also a good movie with Nick Cage about this.

the correct answer for the first is d