*URGENT* What subatomic particle determines the type of element? *
Electron
Neutron
Proton
Mass number

Protons

Explanation:

Forming the nucleus are two kinds of particles: protons, which have a positive electrical charge, and neutrons, which have no charge. All atoms have at least one proton in their core, and the number of protons determines which kind of element an atom is. For example, an oxygen atom has 8 protons.

Sidenote: Hope this helps! :D