Which of the following is a contract that may not be enforceable due to certain reasons?

A void contract is indeed a type of agreement that cannot be enforced by law. This is primarily because it lacks one or more essential elements required for a valid contract, such as mutual consent, a lawful object, and consideration. When a contract is deemed void, it is as if it never existed; neither party can enforce it in a legal setting because it lacks legal standing from its inception.

In contrast, a valid contract meets all the necessary legal elements and is enforceable in court. An illegal contract, while also unenforceable, specifically pertains to agreements that involve activities that are unlawful or prohibited by law. An express contract is a clear agreement often articulated in written form or verbally, and it is typically enforceable as long as it does not fall into one of the categories that invalidate a contract. Thus, the characteristics that define a void contract make it the correct choice for an agreement that may not be enforceable.