An atomic physicist determines that an atom is composed of 8 positively charged particles and has a mass of 15 amu. which is the best conclusion that can be drawn?

Assuming this is a neutral atom, protons = electrons = atomic number
Since there are 8 positively charged particles or protons, there are also 8 electrons and that element's atomic number is 8. Neutrons is atomic mass minus atomic number which is 7 neutrons in this case.This element is therefore oxygen.