应该选B,While（he was） flying in the spaceship,the spaceman saw the Great Wall of China.
句子省略了he was.
A.While flew in the spaceship,应该用正在进行时态,排除.
C.If flew in the spaceship（正确应该是If he fly in the spaceship）,排除.
DTo fly in the spaceship,介词to（是为了,达到的意思）,the spaceman saw the Great Wall of China.是为了 fly in the spaceship 逻辑不对.