The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
Does Palmer explain how the United States government got the information it used to pursue suspected "subversives"?
No, he did not explain how the US government got the information.
What Attorney General Palmer did was explain his actions of 1920 in a document titled "The Case Against the Reds."
In that document, Palmer informed that there were thousands of alien people in the United States territory that were spreading Communist and anarchist ideas supported by Soviet Union's leaders. He added that the method they used to discover this information was classified. So being confidential, that information couldn't be disclosed.