I take it your doing the IA? If so you should have a picture of elements above the question. Oxygen is the only one labeled with 16 as the atomic mass :)
A) oxygen with an atomic number of 8 with an 99.7% of natural abundance.
Explanation:
The atomic mass of an element is the average mass of an element that is measured in AMU. Mass is expressed as Daltons.
Standard atomic weights or mass or the relative isotopic mass of a single isotope is thus tried to that atom. In Order to calculate the atomic mass ad up the protons and neurons. This is usually determined by spectrography.
However, the volumes of two gases and masses are obtained by calculating it with the hydrogen taken as one and thus carbon is taken as a reference to the determination of atomic masses.
