This reverses something the author assumes. The author assumes that enabling children to believe in a better future will help motivate people to work to solve humanity’s problems. But (A) asserts that motivating people to help solve humanity’s problems will enable belief in a better future. That’s not necessary, because even if the motivation doesn’t help people believe in a better future, it can still be the case that enabling the belief in a better future increases motivation to solve problems.

Necessary, because if this were not true — if enabling people to believe better futures are possible will NOT help prevent the loss of motivation that results from pessimism — then we have no reason to think that we “must enable our children to believe that better futures are possible.” Why must we do something that won’t help? The author needs to assume that the solution he advocates for will be helpful.

Not necessary, because whether a particular attitude is in general “better” than another is not at issue. What matters is whether enabling children to think that better futures are possible will help prevent a loss of motivation to solve humanity’s problems. In addition, we don’t know the author’s opinion about how a particular attitude being “illusory” relates to the argument.

Not necessary, because the author does not have to think that humanity’s problems will be “eliminated.” The author can believe that a belief in a better future will motivate others to work toward solutions to humanity’s problems, even if those problems will never be completely gone.

Not necessary, because the argument doesn’t rely on any views about the cause of humanity’s problems. Although the author does believe that improving people’s motivation to solve humanity’s problems might help solve those problems, that doesn’t imply a belief about the original cause of those problems.