for studying BDS which subject we want to take in 9th standard

As far as requirement of BDS is concerned the candidate must have passed Physics , Chemistry and Biology in Class 10th and 12th so in class 9th you must take Science and Mathematics to build a foundation for the same , do check out the link at the bottom of the answer to know the eligibility criterion

• Candidates must have passed 10+2 or equivalent with Physics, Chemistry, Biology/Biotechnology and English as core subjects from a recognized board.

• Class 12 or equivalent appearing aspirants are also eligible to apply for NEET

https://medicine.careers360.com/articles/neet-eligibility-criteria