The element that has 16 neutrons, 15 protons, and 15 electrons is phosphorus. In the periodic table, the number of protons defines the element, so with 15 protons, we are looking at phosphorus (P).
The isotopic notation commonly used is the element’s chemical symbol followed by the mass number (which is the sum of protons and neutrons). For phosphorus, the mass number here would be 15 protons + 16 neutrons = 31. Hence, we can denote this isotope of phosphorus as P-31.
The presence of 15 electrons means that this atom is neutral, balancing the positive charge of the protons with equal negative charge from the electrons.