The answer assumes that the question is about which command help accomplish display directory contents in Unix-like operating systems.
Answer:
The command is ls.
Step-by-step 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).