Final answer:
Wi-Fi is the connection type most likely being used when sharing a smartphone's internet connection with other passengers in a car. It transmits data wirelessly over microwaves in the spectrum of 2.4 to 5.0 GHz, which is suitable for creating a mobile hotspot.
Step-by-step explanation:
When a user in a car with an unlimited data plan on a smartphone opens a wireless connection to share the internet with other passengers, the connection type most likely being used is Wi-Fi. Wi-Fi technology enables the creation of a hotspot that allows other devices such as smartphones, laptops, and tablets to connect to the internet wirelessly.
The Wi-Fi connection uses microwaves in the range of 2.4 GHz to 5.0 GHz to transmit and receive data, making it ideal for this type of internet sharing. Other options like Bluetooth, NFC, and Ethernet are not typically used for this purpose. Bluetooth is commonly employed for connecting devices over short distances, such as headphones to a phone, while NFC is used for very close range communication, and Ethernet requires a physical cable.