John is right on two counts:
1. the CG envelope is not suitable - the graph is distorted, and too small
2. the extrapolation you did by adding 100kg is too small. You are joining two dots very close to each other, so the tiniest of errors will be greatly magnified.
If you want to do the graphical extrapolation more accurately, just add fuel up to the MTOW in the auxes (I know this is more than they can hold). At least then the two dots you are joining together are much further apart and will give a slighty more accurate answer.
I don't really see why Bob Tait doesn't give four multi-choice answers like CASA do. That's why you buy practice exams. He might think he's doing you a favour by making sure you understand what you are doing, but that why you buy text books. Practice exams should be exactly that... a practice at exactly what you are going to get.
In the CASA exams, you would simply pick an answer and try it... if it's right, then it's right! If not, then just work out whether you need a bigger or smaller answer and try again.
Mathematically, 169kg is the correct answer.