The Adoption Deed made by A and B is not legally right. The Adoption Deed suffers from "technical defects".
This seems to me a case of 'customary adoption" where A and B according to customary rules of their "community" adopted C and same day "executed" a Adoption Deed. The formalities necessary for customary adoption in accordance with the rules prevalent amongst certain "communities" are: (i) a declaration of adoption and (ii) "general treatment" of the appointed heir as a son and same needs to be evidentiary established too.
Hence according to the customary rules of adoption the "deed of adoption" could not have any effected unless after its "execution" same day "there was a continuous course of conduct" showing that B treated C as his son and inasmuch from the short brief of yours to show such "association". The brief specifically says that C stayed and enjoyed his biological parents legal heir rights even “after alleged adoption" hence both A and B have failed to make out a case as B fails to show "there was a continuous course of conduct".
I also differ to the opinion of SC advocate @ Anish as in his rely he has erred on 'revocation of adoption deed" and if he ever heard of SC and various State HC's Laws on 'revocation of adoption deed' vis-à-vis "customary laws on adoption" then he should not have said what he said in his reply is my opinion. In short I say "child adoption deed" can be “revoked” in India and there are umpteen case laws.
Also I noticed here (in LCI as in various thread post replies) many SC Advocates say “authoratively” that a child above 15 years cannot be adopted as per Law!!!. May I question these so called SC Advocates including a trial Court practicing female advocate from Maharashtra when you all will sharpen tools of your trade! However this besides a minor reminder be it known that, in Bengal, Banaras and Bihar the courts hold that the boys must be adopted before upanayana*. The same rule holds good in Madras; but there it is further held that if the boy to be adopted is of the same gotra as the adopter, the adoption may be made after upanayana but before marriage, In Bombay a person may be adopted at any age, even after marriage and even after he has had children and he may be even older than the adopter. In the whole of India, a SC / ST / OBC child may be adopted only before his marriage, but in the Bombay Presidency the adoption of a married man and of one having even a child is allowed also among SC / ST / OBC. I suggest these advocates to read and understand Hindu Law before making there so called 'authorative reply" on age of child who can be adopted without asking first "customs and usage S. 3" in the area from where the query originated.
* upanayana; A "Brahmin" child's upanayana must be performed when he is eight years old from conception, that is when he is seven years and two months old from birth. A "Ksatriya's" is to be performed at the age of twelve. The corresponding age for a "Vaisya" is sixteen.