Answer: Gain an electron
Step-by-step explanation:
Electrons carry a negative charge while protons carry a positive charge. Atoms have the same number of protons and electrons which leads to them having no charge because the electrons and protons balance each other out.
If the element acquires an extra electron which is negative, there will now be more electrons than protons in the atom. This extra election's negative charge will not be balanced by an opposing proton charge which would therefore lead to a single negative charge on the atom.