Is the visa that I should apply should be B1/B2? Since this is
equivalent to tourism purpose.
Yes.
Can I fill DS-160 form of my parents on their behalf?
Yes. Please see the official FAQ, question "Can a third party complete the form on my behalf?"
Will the separate DS-160 form need to be filled for my mother and
father or it can be done together?
Each applicant needs a separate form, but the system can automatically reuse and fill up some of the information. Please see the question "I am traveling with my family or as part of a group. Can I create a family or group application?" in the FAQ above. Note that the applications are still evaluated separately, and it is theoretically possible for one parent to qualify for the visa, while another parent wouldn't qualify.
Can the interview be taken in Hindi instead of English? Where can I
specify that?
From my limited experience, the US nonimmigrant visa interviews seem to be typically conducted in the native language of the country/area where the embassy is located. Thus this is likely the case in India as well.
