Question

The anion of which element has a –1 charge?

Answer 1

An anion is a negatively charged ion. An element can become an anion if it takes an extra electron such that it has one more electron than protons.
Atoms would preferentially taken an extra electron so that it can have a full octet and be more stable.

$\sf Cl^-, Br^-, F^-$ are some of the few elements that would have a -1 charge.