The answer is c. He was tired and it was late; it had been a long day. The semi-colon helps distinguish the two separate parts. The part where it says "it had been a long day" doesn't make sense if added by a comma.
In Laika the theme was that it's hard to let something trail you've got grown attached thereto the central idea is About how the doctor will miss the dog after he has grown attached thereto.In Moon Mission the theme is about furthering space advancement and one amongst the central ideas is making a rocket called Gateway to stay storage and put mankind on mars.
