When the U. S. first learned about the Holocaust, they: sent troops to Europe.
asked Hitler to send Jews to the U. S.
placed an embargo on German goods.
did nothing.

I believe it's D.

Explanation:

The US was kept in the dark about the extent of the atrocities committed during the Holocaust. Technically, the US did place an embargo on German good, but that was because of the war they were fighting against the Germans at the time, not really because of the Holocaust itself. After the war and more vivid details about the Holocaust were revealed, the US was heavily scrutinized for not getting more involved.