Aspecific isotope of an element is known to have 15 protons and 16 neutrons. which symbol would properly represent this isotope

When writing the symbol of an element, the correct way is:

ᵇXₐ

Where b is the mass number of the element, which is equivalent to the sum of the number of protons and neutrons, while a is the atomic number, equal to the number of protons. X is the symbol of the element.

In this case, the atomic number 15 belongs to phosphorus, with symbol P. This becomes:

³¹P₁₅

It’s the energy source of stars