Final answer:
A "magic" command in Jupyter notebooks is used for extending the core language with shortcuts, which allows users to execute system-related tasks and interact with the environment outside of regular Python code.
Step-by-step explanation:
In Jupyter notebooks, a "magic" command is used for extending the core language with shortcuts that enable a variety of advanced functionalities. These commands are prefixed with one or two percent signs (% for line magics, %% for cell magics) and are not part of the core Python syntax. They allow users to interact with the underlying system, such as writing files, managing the execution environment, or timing the execution of code. A correct answer to the question posed would be: A. Extending the core language with shortcuts.