What kind of extension is generally not allowed in a listing agreement?

The correct choice relates to the constraints imposed on listing agreements regarding extensions. An override clause that extends for more than six months after the expiration of the listing agreement is typically not permissible. This limitation is in place to protect both the seller and the broker and to foster fairness in the real estate market. Such clauses, if permitted, could indefinitely bind a seller to a broker's commission for a long duration beyond the termination of the original agreement, leading to potential abuse and disputes.

The other options represent different kinds of extensions or communication methods that are generally accepted or negotiable within the sphere of real estate agreements. For example, automated text message updates can serve as a convenient means of keeping parties informed without affecting the contractual obligations. Verbal extensions might occur informally during negotiations but lack the legal enforceability of written contracts and are generally discouraged for clarity and documentation purposes. Lastly, while written extensions are common in real estate practices, they must adhere to specific terms and limitations outlined in the original agreement.