A bilateral contract is one in which:

Option A describes a unilateral contract, not bilateral. In a unilateral contract, only one party makes a promise that the other party can accept only by complete performance. An example is a reward offer that is accepted when someone performs the requested action.

Option C confuses contract type with formal requirements. While some real estate contracts must be written (due to the statute of frauds), this requirement applies to specific contracts, not all bilateral contracts. Many bilateral contracts can be oral and still enforceable.

Option D misstates contract fundamentals. In any valid contract, performance is required, not optional. The distinction is between whether promises are exchanged (bilateral) or only one promise is made (unilateral), not whether performance is optional.