Final answer:
Britain remained determined to fight in WWII not due to a single event, but a combination of factors, one being the significant Battle of Stalingrad which marked the turning point in the war.
Step-by-step explanation:
The one thing that convinced Britain to remain in the war was b) The successful invasion of the Soviet Union by Germany. This question is somewhat misleading as there wasn't one single event that kept Britain in the war; however, the closest option provided would be the Battle of Stalingrad, which was significant as it stopped the German advance into the Soviet Union and was the turning point of the war in Europe, signifying a) It stopped the German advance into the Soviet Union and was the turning point of the war in Europe.
The Battle of Dunkirk was indeed a morale-boosting event for the British, as was their later victory during the Battle of Britain, where Churchill’s determination and speeches reinforced British resolve to continue fighting.