What does the term “basic unit of matter” refer to?

The term " basic unit of matter" refers to an atom.

The atom consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus  contains a mix of positively charged protons and electrically neutral neutrons  except in the case of hydrogen which is considered the only stable nuclide with no neutrons.

The electrons of an atom are bound to the nucleus by electromagnetic force. Two or more atoms can be bound to each to form a molecule.

so whats the answer

you should wear a lead protective shield when having teeth x-rays

hope i

-lavira : )