Concept:The notation −10X describes a particle by its charge and mass number.Explanation:In nuclear symbol notation, the lower number represents the charge on the particle, while the upper number represents its mass number.An electron has a charge of −1 and an almost negligible mass, so its mass number is 0.Thus, an electron is correctly written as −10e.Since the symbol uses −1 and 0, the particle X must be an electron.Answer:C. Electrons