Atomic structure ?
proton, neutron, electron?
atom vs ion ?
mass number ?

atomic structure - The nucleus is at the center of an atom, where most of the charge and mass are concentrated

protons - a stable subatomic particle occurring in all atomic nuclei, with a positive electric charge equal in magnitude to that of an electron, but of opposite sign.

neutrons - a subatomic particle of about the same mass as a proton but without an electric charge, present in all atomic nuclei except those of ordinary hydrogen.

electrons - a stable subatomic particle with a charge of negative electricity, found in all atoms and acting as the primary carrier of electricity in solids.

atom  vs. ion - an atom is the basic unit of a chemical element, and an ion is an atom or molecule with a net electric charge due to the loss or gain of one or more electrons. See also cation, anion.

Mass number - total number of protons, and neutrons in an atom.

Explanation:

i hope this is what you were looking for.

the product of the pressure and volume is also constant

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.