Given these choices, the correct answer is clearly C. The 1950s were a time of stunning economic success.
The 1950s were a time of great prosperity for much of the United States. World War II had done a great deal to hurt the economies of all of the United States' competitors around the world. Conversely, the US itself had not really been touched by the war in the sense that the continental US and its factories, infrastructure, etc. had not been bombed or otherwise destroyed the way that those things had been in Europe and Japan. Because of this huge advantage that the US had (and because of other things like the GI Bill) the '50s was a time of prosperity in the US.
One thing that you should notice about this question is that neither A nor B can be right. This is because they both go together. If there is a downturn in the economy, that means there is rising unemployment. So if one of them were correct, both would be. That is the kind of clue that can help you eliminate possible answers on multiple choice questions.