The correct option is A 3115Y
∙ An element X is symbolically represented as AZX, where 'Z' represents the atomic number and 'A' represents the mass number.
∙ Atomic number = number of protons = number of electrons
∙ Mass number = number of protons + number of neutrons
∙ For the given element, atomic number is 15 and mass number is 31. Hence, it is represented as: 3115Y.