Which type of contract is least likely to be unilateral?

In the context of contract types, a unilateral contract is one where only one party makes a promise or takes an action, while the other party does not have an obligation to perform unless the first party fulfills their promise. An example of a unilateral contract is an option contract, where one party has the exclusive right to purchase something at a specified price within a certain timeframe but is under no obligation to make that purchase.

Looking at the other options, residential resale purchase contracts represent a mutual agreement where both parties (buyer and seller) have obligations, making it far from unilateral. Similarly, brokerage and real property seller agreements involve commitments from both the broker and the seller. Independent contractor agreements also encompass mutual expectations where both parties engage in performance under agreed terms.

Thus, considering the nature of an option contract, it is least likely to be unilateral primarily because of the characteristics and obligations that define the other types of contracts mentioned.