Answer: d. Enable a hotspot on his smartphone.
Step-by-step explanation:
By enabling a WiFi hotspot on his smartphone, Dennis will essentially make his phone a WiFi router (device) which devices can then connect to.
As they connect, Dennis will switch on his mobile data to enable his phone to connect to his network provider's internet service.
Once connected the phone will then share this internet with the devices that are connected to it's WiFi.