Answer is Gen 1. This is USB 3.1 Gen 1 (per SanDisk's website for this item, "SanDisk Ultra Fit™ USB 3.1 Flash Drive").
Some info to help clarify: there is USB 3.0 , and USB 3.1 Gen 1 , and USB 3.1 Gen 2 .
USB 3.0 was renamed USB 3.1 Gen 1, so these two have the same max speed. Max speed of this USB interface is 5 gbps (gigabits per second). That is over 600 MB/s (over 600 megabytes per second). Remember Bits vs Bytes.
Then there is USB 3.1 Gen 2 , with a max speed twice that of Gen 1, so 10 gbps (gigabits per second).
But does the faster speed of Gen 2 really matter for this kind of device? No.
For any device such as a flash drive, the read/write speed inside of it (i.e. flash storage chips inside) is MUCH slower than the flash drive's USB interface speed of even Gen 1 (over 600 MB/s).