Final answer:
The most efficient way to install an operating system on a computer without an optical drive is by using a USB flash drive due to its speed and convenience.
Step-by-step explanation:
If a computer does not have an optical drive, the most efficient method to install an operating system is typically via a USB flash drive. This method allows for a straightforward installation process, as many modern computers support booting from USB drives. Furthermore, USB drives can store larger installation files and provide faster data transfer rates compared to older media like floppy disks or methods like Bluetooth connection; the latter isn't generally feasible for operating system installations due to speed and reliability concerns.