Final answer:
The environment variable that contains the primary shell prompt is PS1, which can be customized to display various pieces of information in the command line interface.
Step-by-step explanation:
The environment variable that holds the primary shell prompt is PS1. The shell prompt is the text displayed in a command line interface to signal that it is ready to accept commands. This variable can be customized to change how the prompt looks, such as including the username, the host name, the current working directory, or other useful information. PATH is an environment variable that specifies a set of directories where executable programs are located. ENV is not an environment variable, but refers to a shell's environment in a broader sense. PSI is likely a typo, as there is no standard environment variable by this name related to shell prompts.