The answer assumes that the question is about which command help accomplish display directory contents in Unix-like operating systems.
Answer:
The command is <em>ls</em>.
Explanation:
The command <em>ls</em> (short for <em>list</em>) 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: (<em>the command "says": display content of a</em>)
Result:
The result is only the directory <em>b </em>because there is no other entry in it.
To display more information regarding <em>b</em>, we can use the many options available for the command <em>ls</em>, like <em>-a</em> (all entries), <em>-d</em> (only directories), <em>-l</em> (long listing format), and so on, e.g. ls -a A (display all entries in directory A, included hidden files).