The set command is used to display or modify the shell variables and functions in the current shell. When used without any arguments, it prints the names and values of all shell variables, including environment variables and user-defined variables, in alphabetical order. The output also includes the shell options and the positional parameters. The set command can be used in any POSIX-compliant shell, such as bash, zsh, ksh, etc123.

The other options are not correct because:

env is used to print or modify the environment variables, not the shell variables. It does not show the user-defined variables or the shell options. It can also be used to run a command in a modified environment45.

env -a is an invalid option for the env command. The -a option is not supported by the env command in any standard or common implementation45.

echo $ENV is used to print the value of the environment variable ENV, not the list of all shell variables. The ENV variable is usually set to the name of a file that contains commands or aliases to be executed by the shell. It is mainly used by the ksh and some versions of bash .

References: 1: How can I list all shell variables? - Unix & Linux Stack Exchange 2: 2.1 Command Line Basics - Linux Professional Institute Certification … 3: set - The Open Group Base Specifications Issue 7, 2018 edition 4: How to set and list environment variables on Linux 5: env - The Open Group Base Specifications Issue 7, 2018 edition : What is the difference between .bash_profile and .bashrc? - Unix & Linux Stack Exchange : ENV - The Open Group Base Specifications Issue 7, 2018 edition