How many valence electrons does an ion with 15 protons have?
We can determine this two ways. The element with 15 protons, meaning it has atomic number 15, is phosphorus. Phosphorus is in group 15/VA. Based on its group number, phosphorus has 5 valence electrons, as do all of the elements in that group.
Another way we can answer this question is to write the electron configuration for a neutral phosphorus atom, which would have 15 electrons: