Question

how did the sinking of the lusitania in 1915 affect world war i?
the united states declared war on germany.
germany continued its policy of unrestricted submarine warfare.
germany restricted its submarine warfare in response to international outrage.
the united states began a policy of unrestricted submarine warfare.

Explanation:

The sinking of the Lusitania led to international outrage. Germany, facing this pressure, restricted its submarine warfare. The U.S. did not declare war immediately after the Lusitania sinking (that happened later in 1917). The U.S. did not start a policy of unrestricted submarine warfare. Germany, due to the outrage over Lusitania, changed its submarine warfare policy (restricted it, at least for a time) rather than continuing unrestricted submarine warfare right after the Lusitania incident.

Answer:

Germany restricted its submarine warfare in response to international outrage.