Final answer:
An unattended installation allows the operating system to be installed with minimal manual input by using an answer file or a script, significantly reducing the technician's hands-on time per computer.
Step-by-step explanation:
To decrease the time it takes to install an operating system on 100 computers, the technician would use a C. Unattended installation. An unattended installation is a method where the installation process is automated, typically through the use of an answer file or a configuration script that contains all the necessary settings for the installation.
This allows the operating system to be installed with little to no manual input, thus significantly reducing the hands-on time required from the technician for each computer.
Options A (Upgrade installation), B (USB installation), and D (Clean installation) all generally require more hands-on interaction for setup or could be limited in availability/capacity when dealing with a large number of computers.