How should I write my answer on " Why marketing"? I got a call from SPJIMR marketing? My first priority was finance but a call from marketing?

The fact that you got a call from Marketing means SPJIMR seems your profile to be more suitable for Marketing than Finance. They have their own parameters of evaluation and you need to trust that part.

Firstly you need to analyze why exactly you have Finance as your first priority. Are you seriously interested in it or just choosing it because you have no prior experience? If you have no Finance background then certificates like CA/CFA might help.

But if you are just casually considering Finance, then I Advise you to look into Marketing because it is what SPJIMR specializes in. You will be losing a big opportunity if you forgo this offer. You will get a good chance to interact with a good peer group and develop an alumni network.

Depending on your preference the answer to ‘why Marketing’ is pretty subjective. Once you are into marketing you will find multiple reasons why you chose marketing. You can conduct research and write a well-informed reason or just simply refer to ready-made answers on CAT forums like Pagulguy for help.