Which of the following is an example of an Internet of Things (IoT) device?

All the provided options are examples of Internet of Things (IoT) devices, which are characterized by their ability to connect to the internet and communicate with other devices.

A smart thermostat is an IoT device because it allows users to control home heating and cooling systems remotely through an internet connection. This connectivity enables features such as learning user preferences over time and optimizing energy usage for efficiency.

A video doorbell also falls under the IoT category as it connects to Wi-Fi, allowing homeowners to receive alerts on their smartphones when someone is at their door. Many smart doorbells come equipped with cameras and two-way audio, enabling real-time communication.

A smart assistant, such as Amazon's Alexa or Google Assistant, connects to various smart devices in the home and can manage tasks like controlling lights, playing music, or providing information. It uses cloud-based technology to respond to voice commands and interact with other IoT devices.

Since all these devices harness internet connectivity and provide smart functionalities, the answer that encompasses all of them being IoT devices is indeed the most comprehensive choice.