Smartphones can be used in IoT (Internet of Things) setups across multiple application categories, typically including:

1. Home Automation: Smartphones can control devices like lights, thermostats, and security systems in a smart home.
2. Health Monitoring: Smartphones can be used to monitor health data, such as heart rate or physical activity, via IoT-enabled devices.
3. Smart Transportation: Smartphones can be used for navigation, tracking, and control of IoT-enabled vehicles and infrastructure.
4. Industrial Automation: Smartphones can be used to monitor and control machinery, equipment, and processes in an industrial setting through IoT sensors.

Thus, smartphones can play a role in 4 key application categories in an IoT setup.

Correct Answer:

(C) 4.