Question

What writer concluded that European society was on the decline after World War I?A. Ernest Hemingway
B. Oswald Spengler
C. Gertrude Stein
D. Karl Barth

Answer 1

D. Karl Barth, I don't really know so check if needed. 

Answer 2

The writer whose Decline of the West proposed that European society had entered the final stage of its existence was B. Oswald Spengler.
100% sure.