Which group was excluded from the united states by the immigration act of 1924?

The act didn't specifically EXCLUDE immigration from any country, but it limited the number of immigrants from any country to 2% of the number already living in the United States.

For instance, if there were 100,000 Russians living in the USA, no more than 2,000 immigrants would be accepted in any one year from Russia.

HOWEVER, the law specifically created to target Eastern European Jews and Asians and limit how many of them could come to the USA. Jewish immigration was severely curtailed to almost nothing, and Asian immigration pretty much stopped dead.

Also, the law prohibited immigration of any people who were inelligible for naturalization. Based on a law from the late 1700s, there were certain countries who's citizens could not become naturalized citizens. These included:

Japan,
China,
The Philippines,
Laos,
Siam (Thailand),
Cambodia,
Singapore,
Korea,
Vietnam,
Indonesia,
Burma (Myanmar),
India,
Ceylon (Sri Lanka)
Malaysia

Thus, people from these countries were specifically prohibbited entry into the USA to emigrate.

Hope that answers your question.