The correct answer is A. voidable at the option of the promisor.
An agreement to do an impossible act is voidable at the option of the promisor. This means that the promisor (the person who makes the promise) can choose to either uphold the agreement or void it. If the promisor chooses to void the agreement, the promisee (the person to whom the promise is made) is not entitled to any damages.
There are a few reasons why an agreement to do an impossible act might be voidable. First, it is generally considered to be unfair to enforce an agreement that cannot be performed. Second, if an agreement is impossible to perform, it is likely that one or both parties did not intend to enter into a binding contract. Finally, if an agreement is impossible to perform, it may be difficult to determine what the parties actually agreed to.
There are a few exceptions to the rule that an agreement to do an impossible act is voidable. For example, if the parties agree to do something that is impossible at the time of the agreement but becomes possible later, the agreement may be valid. Additionally, if the parties agree to do something that is impossible in the ordinary course of events but is possible with extraordinary effort, the agreement may also be valid.
Ultimately, whether an agreement to do an impossible act is void or voidable depends on the specific facts and circumstances of the case.