15 electrons are contained in an atom: a) arsenic b) chlorine c) scandium d) phosphorus

To answer this question, we need the periodic table of chemical elements of D.I.Mendeleev. Of course, this is a periodic system, you must be able to use and know the periodic law. So let me explain in the periodic table we will find an element with serial number 15. And this will be phosphorus. So, phosphorus has 15 protons in the nucleus of an atom and 15 electrons in orbitals, respectively. Therefore, this is point d) from the explanation to the question.