Natural selection is referred to as "Survival of the Fittest". What does being the most "fit" mean

The best to adapt.

or the one with the set of skills vital to the situtation.

If were in the mountains, the one most "fit" will live. That will mean haveing a specfic body type.

If we could only eat nuts, i would die off. I have an allergy. My body is not "fit", so the "stronger" ones will prevail

Explanation:

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.