The atomic mass of an element is

a) the sum of the number of protons and electrons in one atom of the element
b) twice the number of protons in one atom of the element
c) a ratio based on the mass of a carbon-12 atom
d) a weighted average of the masses of an element's isotopes

C) A ratio based on the mass of a carbon-12 atom

Explanation:

For each element, there is a characteristic number of protons, this number represent a type of atom and its called the atomic number

On the other hand we have the number of neutrons, and for one element which difers only on the number of neutrons, we have different isotopes from one element.

Together, protons and neutrons define the mass number

One property linked with the mass number, is the atomic mass, which is formerly expressed as "uma" (atomic mass unit)

By definition, one atom of carbon-12 (with 6 neutrons) has an atomic mass of 12 umas

The answer is "C) A ratio based on the mass of a carbon-12 atom"

Explanation:

The atomic mass of an element is the weighted average mass of its natural isotopes. Therefore, the atomic mass of an element is not an integer. The weighted mean means that not all isotopes have the same percentage. In other words, the atomic masses of the chemical elements are usually calculated as the weighted average of the masses of the different isotopes of each element, taking into account the relative abundance of each one.

Amu (u) is often used as the unit of measurement. Where u.m.a are acronyms that mean "Atomic Mass Unit".  It has an approximate value of 1,6605 · 10⁻²⁷ kg which corresponds to one twelfth of the mass of the carbon-12 atom.

So, the answer is "C) A ratio based on the mass of a carbon-12 atom"

see earlier answer