Can I pursue MBA after completing class 12th? Is it necessary to complete UG in order to pursue MBA?

No, it's not compulsory. But you need to appear for entrance exams like SET, IPM, NPAT, etc and score well to be eligible of taking admission into reputed B-schools like Symbiosis Pune, IIM Indore, NMIMS Mumbai, etc, for their integrated 5-years MBA programme(BBA + MBA). For pursuing the normal MBA

Yes it is compulsory to be graduated for MBA admission.

There is an integrated MBA (5 years) in IIM Indore called IPM, directly. You can apply for MBA after 12. But, you have to give entrance & there are 120 seats.

Yes, of course. You need to complete your graduation before joining B-SCHOOLS for MBA.

You can't do after 12th. You need to have a graduation.