CMAT

You can go for CAT exam.

CMAT is better as there are decent colleges, which accept it like SIMSREE, GIM, etc. CMAT is also comparatively easier than other exams like CAT, XAT, etc.

You are eligible to apply through CMAT. Please check DTE website www.dtemaharashtra.gov.in for latest cut off updates.

Check this link for the same-
http://www.entrancecorner.com/bschool/cmat-syllabus/

Opt for both, as well as CAT so you can have various opportunities for admissions.

CMAT and MAT are almost same. CMAT has GK pattern too, so makes it more vivid and dynamic.

Dear Anil Kumar,
Prefer Arihant as the first option and later Disha. Apart from that, your own understanding and preparation for CMAT is necessary along with self-reliance. All the best.

CMAT stands for Common Management Admission Test which is conducted by AICTE and ATMA stands for AIMS Test for Management Admissions which is conducted by AIMS.
Both of these are national level entrance exams but CMAT is more preferable as CMAT is accepted by more college as compared to ATMA.

Dear Anil Kumar,
Try to get a good score in both the entrance examinations. You will get the best college. Also, try to focus only on college and their score requirement for admission rather than the difficulty level of the tests. All the best.

On the first place, always give preference to regular MBA. Going with your query, if external MBA means distance learning then you don't seem to give any exam. You can directly enroll for the course from the institute you are willing to take admission from. This is what I know. Kindly, confirm before considering it.