Question

What action did the Native Americans take after the war?

They gave up their lands to the American government.

They signed additional treaties with the Confederacy.

Many Indian tribes raided the Northern states in retaliation against the Union.

Native American tribes nullified the treaties they held with the American government.

Answer 1

Likely "They gave up their lands..."

Explanation:

During the Civil War the Union Army didn't have many resources left to fight Indians but there were still problems. After the Civil War Indians got all their business.

The expansion of the Railways in the 1860s brought a wave of settlers to the West. Gold discoveries brought more white men. The U.S. Government sign treaties with the Indians only to find them ignored by the settlers and miners.

Andrew Jackson's expulsion of the Indians East of the Mississippi in the 1830s meant that problems with the Indians were in the West.

Wholesale slaughter by commercial hunters in the 1870s destroyed the herds of buffalo that the Natives in the Great plains survived on. After 1876 Indian resistance slackened. Life wasn't better on the reserves but it was possible.

As for the other answers: The Confederacy ceased to exist so there was no making Treaties with them so #2 is incorrect.

#3 seem incomplete as Indians certainly raiding throughout the 1800s but it diminished over time as the Army actively suppressed hostile Indians, particularly after the Civil War.

#4 It was pointless for Indians to nullify treaties as everyone else was nullifying them faster. Eventually the Treaties were all the Indians had.