@Bishnu15 bro, both the phones are addressing a Budget-requirement. It depends on your requirement and usage.
Micromax Bharat 1: Cost Rs 2200/-
Feature Phone (not a smartphone)
Has just 512MB of RAM.
According to me, this is a big no-no. Even if you buy, you wont be able to use it well with so less RAM.
Karbonn A40 Indian: Cost Rs 2899/-
Smartphone with 4.0 Inch Screen, although WVGA - will give you very poor viewing angles
Hosts 1GB of RAM and 8GB of internal storage - excellent for a budget phone. Will address minimal basic requirements
If you see the price difference, Karbonn is a much better deal.
As a disclaimer, both these brands have a very bad reputation in terms of sustainability. So think well before you proceed.
I had asked this question earlier, if we may buy it online and can still avail the offer with existing airtel SIM. It is best to contact Customer Care and confirm. It is available on amazon for Rs 2899 and the official page lists airtel's offer too. I guess it should be valid if purchased from amazon
(open the link and scroll down for airtel offer).
Amazon Link:
https://www.amazon.in/dp/B07377RPMP/?tag=dreamdth-21