The type of contract that has been formed between Maria and John is a bilateral contract. A bilateral contract is one in which both parties exchange promises that create mutual obligations. In this case, Maria promised to pay the advertised price, and John promised to sell the house. This mutual exchange of promises establishes a binding bilateral contract, placing obligations and granting rights to both parties. Option A and Option B describe different types of contracts that are not applicable here, while Option D refers to unilateral contracts, where only one party makes a promise contingent upon the performance of an act by the other party. Option E is incorrect because Maria's offer and John's acceptance formed a valid bilateral contract, not merely an invitation to treat. 

Key Point: A bilateral contract involves mutual promises between two parties, creating reciprocal obligations and rights. This type of contract is commonly used in agreements for the sale of goods or services, where both parties are bound by their respective promises.