Is transfer of title from agpa valid

B is the titleholder (owner) of the subject property even if he has executed Agreement to sell and GPA in favour of C.

C has neither any title, interest, claim, authority or title of the property under sale/ purchase.

Does Person C has rights to execute sale deed to me ?

Usually, in a Sale agreement cum GPA, the agent of the GPA will have power to sell/mortgage/develop or any transfer.  If such a clause is not there, you cannot purchase the property.

No, C has no title/ownership rights over property so if you want to buy the property then you require B's signatures on sale deed.

As C did not get the sale deed in his favor, he is not the owner of the property. He is merely a holder of an agreement for sale, which does not transfer title to him. Heynce you have to obtain a sale deed executed by B as owner and C as agreement holder and confirming party. get the sale deed drafted by an experienced Advocate and get it registered. You also should check the property card of the property and satisfy that B still continues as Owner.