Concept:The atomic number of an element is the number of protons found in the nucleus of one of its atoms.Explanation:Aluminium is represented by the symbol Al.Its atomic number is 13, which means each aluminium atom contains 13 protons in its nucleus.This value is a fundamental property that identifies the element and determines its position in the periodic table.Therefore, among the given options, the correct atomic number is 13.Answer:A. 13