The species that is formed when a molecule gains an electron is called an isotope. an isomer. a charge mass ratio. an anion. a cation.

            The species that is formed when a molecule gains an electron is called an Anion.

Explanation:
                   An Anion is any specie which contains negative charge due to gaining of electron or electrons.

Example:
                                         Cl₂        →       2 Cl⁻

In above example Chlorine has gained one electron per atom and is converted into Cl⁻.

Each options are incorrect because,

Isotopes have same number of protons but different number of neutrons.

Isomerism is a phenomenon in which compounds have same molecular formula but different structural formulae.

Charge to mass ratio is employed in instrumental techniques for calculating charges and masses of subatomic particles.

Cations are formed by removal of electron.

Cr(oh)3 -> cr(3+) + 3oh-