What was Civil Defense supposed to do for Americans? to teach Americans about the realities of atomic attack

to show Americans that an atomic attack was unlikely

to calm Americans' fears of atomic weapons

to help Americans build their own fallout shelters

I’m not sure but I think option D;
To help Americans build their own fallout shelters

Explanation:

United States civil defense refers to the use of civil defense in the history of the United States, which is the organized non-military effort to prepare Americans for military attack.