Answer:
Based on the options provided, the best device for your friend to purchase and configure would be a Bluetooth card reader. This type of device connects wirelessly to a mobile phone or tablet through Bluetooth technology, allowing your friend to accept credit card payments from customers using his mobile phone.
A memory card reader is a device that is used to read and transfer data from a memory card, such as an SD card, to a computer or other device. An IR reader is a device that reads data using infrared technology, which is typically used for short-range communication. A DB-9 reader is a type of connector that is used to connect devices using a 9-pin serial connection. It is not typically used for accepting credit card payments.
Overall, a Bluetooth card reader would be the most appropriate device for your friend to use in order to accept credit card payments from customers at the farmer's market using his mobile phone.
Step-by-step explanation: