A) He worries how children will be affected since one of the Challenger crew was a teacher, is the best answer because it explains why Reagan addresses “the schoolchildren of America” specifically in Paragraph 4 of the Space Shuttle text.
In the paragraph, Reagan addresses the children and mentions that it's hard to understand why this accident happened, but that it's all part of the process of exploration and discovery. He goes on to say that the future belongs to the brave and that the Challenger crew was pulling us into the future. He also mentions that it's important to follow them.
Reagan mentions the Challenger crew and the future which implies that he wants to assure that children will remain interested in NASA and help America one day win the Space Race, but it's not the main reason why he addresses the children. But what makes this the best answer is that Reagan addresses the children directly after mentioning that one of the Challenger crew was a teacher. this mention is a reference to the fact that the teacher was going to be teaching from space, and that the children were excited to see that. He implies that he's worried about how the children will feel about the accident, and wants to assure them that despite the tragic outcome, the spirit of exploration and discovery will carry on.