Hey,

You can pursue any course in bachelors for taking up the IAS exam. However, as you are already clear, that you want to opt for Botany/Zoology as an optional subjects, then, it would be best if you figure this out and pursue your bachelors in this field only. Optional subject plays a very important role in IAS exam. Even if you are confused in this, you can better go for B.SC (Zoology, Botany, Chemistry). Because in this you would be able to study both Botany and Zoology and at the end you can choose subject of your choice as an optional.

Know more about IAS Exam over here: https://competition.careers360.com/articles/upsc-ias