D. rift valleys.
Trenches occur at convergent oceanic plates; fossils are extremely old bones, bodies, or footprints, which have gotten so old the intervening years have turned them into rock; U-shaped valleys are valleys which are shaped like the letter U, and are usually formed by glaciers, which tend not to occur underwater very often.
SOS:
<u><em>Sierra Leone</em></u> is the answer!
<em>Hope this helps!!</em>
I guess the answer to the blank will be:
During the second Red Scare public and government suspicious of Communism led to the blacklisting of people in various industries.