If you assume something about someone, you could be acting on false information and that can lead to trouble.
The answer is their. Their is when something belongs to them.
They're: They are
There: Over THERE
Their: belongs to someone
Answer is D
You read the book correct? if so just write who they are, What they wanted and if you didn't read the book skim through it and read some paragraphs. What was the problem? Now how did they solve that problem? The book has all the answers.