Question

the united states formally entered world war ii following the japanese attack on
nanjing

indochina

pearl harbor

the philippines

Explanation:

The United States formally entered World War II after the Japanese attack on Pearl Harbor on December 7, 1941. The other options: Nanjing was a site of a massacre by Japanese forces but not the trigger for US entry; the Philippines and Indochina (a region) were involved in WWII but the attack on Pearl Harbor directly led to US declaration of war.

Answer:

Pearl Harbor (the option corresponding to "Pearl Harbor")