First of all opt botany because you are M.Phil in Botany. Second don't opt physics, reason is that you are Biological Science student through out your carrier so you havn't good base of Mathematics to understand physics. Chemistry is wise option because in M.Phil you have studied chemistry( spectroscopic studies, analytics studies,organic chemistry) in depth,so half of your syllabus already completed.
so in my opinion opt Chemistry.

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“Never leave that till tomorrow which you can do today.”
Benjamin Franklin