Question

the atomic mass of an element takes into account the mass of each isotope and...
the atomic number of each isotope.
the number of neutrons in each isotope.
how often each isotope occurs in nature.
the number of protons in each isotope.

Explanation:

The atomic mass of an element is a weighted - average of the masses of its isotopes, where the weights are the natural abundances (how often each isotope occurs in nature) of the isotopes. Atomic number, number of neutrons, and number of protons do not directly factor into the weighted - average calculation for atomic mass in the way that natural abundance does.

Answer:

how often each isotope occurs in nature.