The answer u r looking for is- thylakoids. ;)
D. The pathogen had been weakened
It is definitely not a primary source: this would be a book itself.
Now, is it a tertiary source? I don't think so: the author of the review should have read the book and should only be referring to this book in the review.
I think it's a secondary source.
If I'm correct, it would be the microbes.