Read this sentence from the passage. 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? people must always be patriotic. optimism is important to success. optimism must be supported by facts. people tend to be pessimistic.

Optimism is important. If people do not try with the firm belief that will succeed, then there is only one other alternative. They will try but they do not believe they will succeed. It's a reflection of the American Attitude which the entire outside world believes is great about America. It is that "Can do" belief. Everything Roosevelt uttered while he was in office reflected this style of life.

There's no mention of the need for facts. There is no mention of people being pessimistic. There's no mention even of patriotism which is likely the second best answer. There is  only this guiding principle of Can Do (though it is not explicitly mentioned either. But Can Do and Patriotism and Success are closely connected.