When a bromine atom gains an electron, it becomes a negatively charged ion. This species is known as a bromide ion. Therefore, the correct answer is b. bromide.
To explain further, bromine is a halogen found in Group 17 of the periodic table. It has seven electrons in its outer shell and needs to gain one more to achieve a stable electron configuration, similar to the noble gases. When bromine gains that additional electron, it transforms into a bromide ion, which is represented as Br–. The additional negative charge signifies that the atom has gained an electron, hence the term ‘bromide’ is used to refer to this ion.