The directory "a" contains its subdirectory "b" and there is no other entry in the directory "a". You want to display "the contents of directory a are: b". The contents are to be displayed using "ls" command. Which command helps you accomplish that?

The answer assumes that the question is about which command help accomplish display directory contents in Unix-like operating systems.

The command is ls.

Explanation:

The command ls (short for list) displays a variety of important information in different ways regarding any directory contents. Because of this, it is probably one of the most used commands in Unix-like operating systems.

Any directory can contain directories and files of different sizes, recently created/modified, attributes like permissions for being accessed, and, with this command, we can see all this information by size, chronologically, by owner, and/or by many more ways.  

In the question, we can accomplish to list that the directory "a" contains its sub-directory "b" and no other entry using the next line of code:

Command-line 1: (the command "says": display content of a)

Result:

The result is only the directory b because there is no other entry in it.

To display more information regarding b, we can use the many options available for the command ls, like -a (all entries), -d (only directories), -l (long listing format), and so on, e.g. ls -a  A (display all entries in directory A, included hidden files).