Looking at your location and your statement that
"Found this question in a book "
I suspect that you are undergoing flight training of some kind, possibly with OATS (or whatever they are called now). I could of course be wrong but that is the basis of my answer to your question.
What is the best CofG position with a stuck stabilizer, and why ?
The answer depends on what your flight condition was at the time the stabilizer became stuck.
In cruise flight for example, the aircraft would be trimmed for a low pitch attitude / angle of attack and gear and flaps up. In order to land you would need to extend the gear and flaps and assume a higher angle of attack / pitch as you decelerated the aircraft to landing speed. These actions would generate a nose down pitching moment, which would normally be countered by adjusting the stabilizer trim.
The stabilizer stuck in cruise position would not only prevent you from trimming out the pitch-down moments, it would actually increase them. A forward C of G position would make the problem even worse. So an aft C if G would be the most favourable in this scenario.
The ATPL CQB contains (or did for many years) a question concerning how best to achieve a landing with the stabilizer stuck in the cruise position. The best answer was a combination of aft CG, not much flap, and higher than usual landing speed.