An ion is an atom or a molecule that has lost or gained one or more electrons. So the correct option is A.
What is an ion?
An atom or molecule which has a charge is referred to as an ion. Usually, the number of electrons and protons in an atom is equal. While electrons are present in the atomic shells, protons are present in the nucleus.
Because, the electrons are negatively charged and the protons are positively charged, the loss of one or more electrons would mean that the atom will now be positively charged. This will be called a positively charged ion.
Also, the gain of one electron would mean a gain of an additional negative charge. So the atom will now be negatively charged and will be called a negatively charged ion.
This is true for an atom individually or an atom which is part of a molecule. In that case loss or gain of one electron would result in the development of a positive or negative charge on the molecule.
Therefore the correct option is A.
Read more about ions, here
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