Final answer:
The source of law that requires the acceptance to be a "mirror image" of the offer in order to form a contract is common law.
Step-by-step explanation:
The source of law that requires the acceptance to be a "mirror image" of the offer in order to form a contract is common law. Under common law, the acceptance must be an exact and unconditional acceptance of all the terms of the offer for a contract to be formed. Any deviation or modification from the original offer would be considered a counteroffer and would require acceptance from the original offeror in order to form a contract.