In the total and in the beginning, you will affect the SEO with the change, but the question becomes how much.
Another consideration is brand equity and whether you would lose any. If, the only reason for changing is to show that one is under the umbrella of the other, I am not sure of that as a good strategy. Why not just put something on the site that makes that clear?
If however, you are having dupe content issues, want to consolidate staff, want to try and boost one site, etc. then you can 301 A to B. With the tradebriefs having higher authority, I would do as you suggest doing and go to it. So, indiaretail 301's each url to the appropriate url on tradebriefs. Not 301 indiaretail.com to trade briefs.com.
The last two with subdomain and sub directory is not the way to go IMO. The subdomain is its own domain and does not share linking, etc. With the way you are showing a single sub directory, that would kill a ton on your site and you would waste a lot of your linking.
The 301 url to url is the way to go.
Hope that helps,
Robert

