Question

This great Nation will endure as it has endured, will revive and will prosper. Which concept does Roosevelt most likely intend to convey in this sentence?
Optimism must be supported by facts.

People tend to be pessimistic.

People must always be patriotic.

Optimism is important to success.

Answer 1

Which concept does Roosevelt most likely intend to convey in this sentence?  

The answer is, Optimism is important to success.

When Roosevelt reinforces the fact that this nation will endure as it has endured, he does it to tell his people that no matter how it looks today our nation will prosper, no matter how bad it appears to be, this nation has always stood up. American Manifest Destiny, though it refers to expanding his lands by providential order, also gives Americans a providential pull, a lift up, that always takes them forward. Roosevelt is aware of this, but his people has forgotten it, and he reinforces this truth to lift up the moral of the American people.

Answer 2

The correct answer is Optimism is important to success, this can be confirmed if we take a look at the rest of the excerpt  from his inauguration speech:

"This great Nation will endure as it has endured, will revive and will prosper. So, first of all, let me assert my firm belief that the only thing we have to fear is fear itself—nameless, unreasoning, unjustified terror which paralyzes needed efforts to convert retreat into advance. In every dark hour of our national life, a leadership of frankness and vigor has met with that understanding and support of the people themselves which is essential to victory. I am convinced that you will again give that support to leadership in these critical days."

This inauguration address was given by President Roosevelt at a very difficult time for the American people;  the depression had reached its lowest point and Americans were really concerned about the situation, Roosevelt intended to convey the message that although there were huge material concerns the optimistic spirit of the people will be crucial for them to succeed.