Final answer:
The sentence indicating the occasion for Franklin D. Roosevelt's "Day of Infamy" speech is the one where he asks Congress to declare war in response to Japan's attack on Pearl Harbor on December 7th, 1941.
Step-by-step explanation:
The sentence from Franklin D. Roosevelt’s “Day of Infamy” speech that indicates the occasion is: “I ask that the Congress declare that since the unprovoked and dastardly attack by Japan on Sunday, December 7th, 1941, a state of war has existed between the United States and the Japanese Empire.” This sentence succinctly exemplifies the urgency and gravity of the situation by pinpointing the specific event that led Roosevelt to address Congress—the Pearl Harbor attack—and directly requests an action from Congress, which is to acknowledge the existence of a state of war.