What tort has the Defendant committed against the ISP for sending unsolicited e-mails?

The correct answer is that the Defendant has committed trespass to chattel against the Internet Service Provider (ISP) by sending unsolicited emails. Trespass to chattel occurs when an individual intentionally interferes with another's lawful possession of a chattel (an item of personal property). In the context of sending unsolicited emails, the Defendant is effectively using the ISP’s resources (such as bandwidth and server space) without permission, which interferes with the ISP's ability to provide services to its customers.

Unsolicited emails, commonly known as spam, can consume network resources and can negatively impact the performance of the ISP’s service. This type of interference is what establishes the tort of trespass to chattel, as it violates the rights of the ISP to control its property and the use of its services.