B.Sc Botany

Please refer to the link provided below for details on how to calculate best of four percentage for B.Sc at DU.
DU Science Admission Cut-offs:
How to calculate the best of four percentage:
https://www.shiksha.com/science/articles/du-science-admission-cut-offs-2016-how-to-calculate-best-of-four-percentage-blogId-12765

In-order to get admission in Ranchi University for Post Graduate course in Botany, you have to clear the entrance exam organized by the university.
Admission Eligibility: B.Sc/ B.Sc (Hons.) with 55% aggregate marks.
For More Info please visit: http://www.ranchiuniversity.ac.in/2014-03-31-05-27-00/2014-04-02-06-49-40/botany.html

Yes, you can apply but you must have mathematics as a subject in your 10+2 course.
Thank you.

Please refer to the link below for eligibility criteria:
https://www.shiksha.com/teaching-education/exams/Kerala-set.

Yes, you can go for M.Sc in forensic Science and criminology, but only after after clearing it's entrance exam.

You have a wide variety of choices, you can go for specialisation in botany, agricultural science, forestry, biotechnology, environmental science, nutrition, chemistry , biochemistry. There are few other options as well which include the course related to the given subjects.

Take a look for Professional courses after B.Sc. -
M.Sc (physics, chemistry, maths, biology, botany, geology and zoology, Applied Biology, Bio-chemistry, Bio-technology, Molecular Biology, Genetics, Environmental Biology)
Post Graduate Courses - You can apply for Research based institutes for PG courses. Ph.D. go for integrated Ph.D. programme or prepare for csir NET. IISER - The value of IISER is really high in Science field. Regards.