Explanation
The apt-get subcommand that installs the newest versions of all currently installed packages is dist-upgrade.
The dist-upgrade subcommand performs the same function as the upgrade subcommand, which is to install the latest versions of the packages that are already installed on the system, but it also intelligently handles the dependencies and removes the obsolete packages if necessary. The dist-upgrade subcommand is useful when upgrading the entire system to a new release or distribution12.
The other options are either invalid or do not perform the desired task. The auto-update subcommand does not exist, and the update subcommand only updates the list of available packages from the repositories, but does not install any packages. The full-upgrade subcommand is an alias for the dist-upgrade subcommand, so it performs the same function, but it is not the standard name for the subcommand. The install subcommand installs new packages or specific versions of packages, but it does not upgrade all the currently installed packages. References:
* Linux Essentials - Linux Professional Institute Certification Programs1
* Exam 101 Objectives - Linux Professional Institute2
* APT-GET Command in Linux {Detailed Tutorial With Examples} - phoenixNAP3
* How do I get help on apt-get's install subcommand?
* APT Cheat Sheet | Packagecloud Blog