Final answer:
The statement is true; the average atomic mass of an element is the weighted average of its naturally occurring isotopes, considering their percent abundances.
Step-by-step explanation:
The statement that the average atomic mass is a weighted average that reflects the natural percent of the all the isotopes of an element found in the universe is true. The calculation of the average atomic mass takes into account the percent abundance of each isotope as a fraction. This means that the average atomic mass listed on the periodic table or in any reference table is not just an average of isotope masses but a weighted one based on how much of each isotope exists naturally.
For example, if we use carbon, which has two main isotopes, ¹²C with a percent abundance of 98.89% and a mass of exactly 12 amu, along with ¹³C which has a percent abundance of 1.11% and a mass of 13.003355 amu, the average atomic mass is calculated by multiplying each isotopic mass by its fractional abundance and then adding these products together.
The same principle applies to all elements and their isotopes, resulting in the atomic masses displayed in the periodic table and used universally by chemists and scientists.