Hello,


Its really impressive that you are so serious for the preparation of the exam and already implementing strategy.Now, coming to question first of all let me tell you zoology as optional in UPSC is good option but do keep in mind that their syllabus is what you need to cover very deeply and its not becesaey that it will be covered by your graduation course.
I would suggest you to go through UPSC official website and check and observe the entire syllabus and also past year question as that would be enough to decide which subject to choose.

I hope this helps.