The correct answer is A) He decided to run for a third term as president.
The unprecedented decision that FDR made in 1940 was that he decided to run for a third term as president.
On July 18, 1940, Franklin D. Roosevelt accepted the Republican Party nomination to contend for a third term of the presidency of the United States. Roosevelt thought that it was his duty to continue to be of service to the citizens of America.
Well, he did not only accepted to run for a third term but for a fourth term too. In 1944 he again accepted the nomination to contend for another term.