Which of the following wireless technologies would most likely be used to make a purchase...

The CompTIA Tech+ Study Guide defines:

“Devices that use radio frequency (RF) transmit a very limited range, typically only a few inches or so. This technology is fairly common in mobile payment applications. Because of the short distances involved, a related term you’ll see is near-field communication (NFC).”

NFC allows devices such as smartphones to securely transmit payment data over short distances—precisely the method used in vending machine purchases.

Verified Reference:

Chapter 2: Peripherals and Connectors — “Wireless Connections: NFC” (p. 99).