Final answer:
The day of the Pearl Harbor attacks is considered 'infamous' because it entailed significant loss of American lives from an unexpected attack, ended American isolationism, and signified the nation's entry into World War II.
Step-by-step explanation:
President Franklin D. Roosevelt recognized the day of Pearl Harbor attacks as 'infamous' because of all the reasons mentioned in options A, B, C, and D. Primarily, it was a moment of significant loss for the United States. On December 7, 1941, many American soldiers and civilians died due to the unanticipated and unprovoked attack by the Japanese. This event also marked the end of American isolationism and signified the start of the country's active involvement in World War II. Therefore, the moniker 'infamous' symbolizes the shock, the loss, and the transition that this day represented in American history.
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