The correct answer is 18.Concept:Atomic number: The number of electrons or protons in an atom.Mass number: The number of protons and neutrons taken together.Isotopes: The elements with the same atomic number but different mass numbers due to the different numbers of neutrons.Explanation:Group number: It tells how many valence electrons are present in the outermost shell of the atom.Element X has group number 15. So, the no. of valence electrons is 15-10 = 5.Period number: It tells how many shells are there in the atom. Element X belongs to the 3rd period. So, there are 3 shells. Electronic configuration of Element X = 2, 8, 5.Its atomic number is 15. Therefore, there are 15 electrons and 15 protons in it. The mass number of its isotope is 33.No. of neutons + no. of protons = 33No. of neutons + 15 = 33No. of neutrons = 33 - 15 = 18