CMAT

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.

Check these links for the same:
http://blog.oureducation.in/top-coaching-centres-for-mba-entrance-in-pune-maharashtra/
https://testprep.shiksha.com/mba-entrance-exams-pune-categorypage-14-47-1-0-0-174-114-2-0-none-1-0

You can get admission through any of XAT, MAT, CAT, CMAT and GMAT score. You just need to check for the exam scores college's accepting, you are looking for getting admission.

Common Management Admission Test (CMAT) is a national-level computer-based management entrance test. The test comprises of questions related to quantitative techniques and data interpretation, logical reasoning, language comprehension and general awareness. You need to focus on these during your preparation. Besides books / study material (even newspapers) you need to have a small group of students who can ask each other questions and help prepare each other. Appearing for mock tests also helps a lot.