Section 20 of the Specific Relief Act, 1963 lets a party get a contract performed through someone else and then recover the extra cost from the defaulting party, but only if certain conditions are met. The question asks whether A can recover the extra ten lakh from B, and each option needs to be checked against those conditions.
Only the missing prior notice, not the general availability of substituted performance, not B's breach on its own, and not any need for a prior court declaration, is what actually bars A's recovery here.
Therefore, the correct answer is A cannot recover the cost because A did not give B prior notice.