Answer:
/dev/sdc1
Step-by-step explanation:
The udev system of Linux will recognize SCSI/SATA drives in the /dev/sda..., /dev/sdb..., /dev/sdc... series depending on the order of attachment.
If at the time of mounting the first SCSI/SATA drive, It believes the /dev/sda... series is not in use in the system it will mount that drive and its partitions as /dev/sda1, /dev/sda2, /dev/sda... and then the next SCSI/SATA drive and its partitions as /dev/sdb1, /dev/sdb2, /dev/sdb... and so on.
In this case, since two SATA hard disks are already attached, those first two would have taken up the /dev/sda... and /dev/sdb... series leaving the USB flash memory drive, another SCSI/SATA drive with the /dev/sdc... series.