Which of the following will allow the easiest and fastest way to share a single file between two modern smartphones without joining the same WiFi network?

Using a Micro SD card would involve physically removing the card from one phone, inserting it into another (if compatible), transferring the file, and then reversing the process, which is cumbersome and not the easiest or fastest.

Bluetooth is a short-range wireless technology specifically designed for peer-to-peer device communication. It's built into virtually all modern smartphones, easy to enable, and doesn't require an internet connection or a shared Wi-Fi network, making it ideal for quick file transfers between nearby devices.

A USB connection would require a cable, and potentially an adapter (e.g., USB-C to USB-C, or USB-OTG), and might involve specific file transfer modes, making it less convenient than a wireless option like Bluetooth.

Infrared is an outdated technology for file sharing between phones; while it existed, it is slow, requires line-of-sight, and is not present on modern smartphones, making it impractical.