Final answer:
NFC (Near Field Communication) is the mobile device connection being used for a user to pay for a retail purchase by placing a smartphone next to the merchant's reader.
Step-by-step explanation:
The correct answer is NFC (Near Field Communication).
NFC is a technology that allows devices to communicate wirelessly over a short distance, typically less than 4 centimeters. It enables secure communication between the user's smartphone and the merchant's reader, allowing for contactless payments.
Unlike Bluetooth, which requires pairing and a longer-range connection, NFC does not require any setup and is specifically designed for close-range interactions like making payments or transferring small amounts of data.