Answer
Correct option is (a): forms a negatively charged ion.
Work Step by Step
When an atom gains or loses an electron it becomes an ion in order to attain the stable configuration. If an atom gains electron, it will become negatively charged ion or anion. The atomic radius of the orbit increases because now the atom has more electrons around.
An atom cannot form positively charged ion, on gaining electrons as the negatively charged particles are increased in the atom, hence b option is incorrect.
An atom can form covalent bond,only when it shares electrons with oppositely charged ion. Therefore, the option (c) is incorrect.
Ionic bond is formed through an electrostatic attraction between two oppositely charged ions. Therefore, the option (d) is incorrect too.
As the mass of electron is negligible, it cannot add mass to the atom. Therefore, the option e is also incorrect.
Hence only option a is correct.
