Question

roosevelt’s executive order no. 9066 was based on
evidence that all japanese americans were plotting to fight the us within its borders.
past experiences of japanese americans causing civil unrest in the united states.
the assumption that japanese americans posed a threat to national security.
written reports that japan was recruiting american citizens to spy for them.

Explanation:

Executive Order 9066, issued during WWII, authorized the internment of Japanese Americans. There was no concrete evidence of widespread plotting, civil unrest caused by them, or confirmed written reports of mass spying. The order rested on the unfounded assumption that this group posed a national security risk due to anti-Japanese sentiment and wartime fear.

Answer:

the assumption that Japanese Americans posed a threat to national security.