Recall that an anion is an ion that has gained one or more electrons.
Since electrons carry a negative charge, gaining electrons results in an overall negative charge on the ion.
Therefore, an anion always has a negative charge, distinguishing it from cations, which have positive charges.
The term 'variable' charge does not apply here because anions consistently carry a negative charge, although the magnitude can vary depending on the number of electrons gained.
Hence, the correct description of an anion's charge is negative.