The element lanthanum has two stable isotopes, lanthanum-138 with an atomic mass of 137.9 AMU and lanthanum-139 with an atomic mass of 138.9 AMU.

From the atomic weight of La = 138.9, one can conclude that:

A. there is an isotope of lanthanum with an atomic mass of 138.9

B. lanthanum-139 has the highest percent natural abundance

C. lanthanum-138 has the highest percent natural abundance

D. both isotopes have the same percent natural abundance

A. there is an isotope of lanthanum with an atomic mass of 138.9

Explanation:

By knowing the different atomic masses of both Lanthanum atoms, we can not tell anything about their occurence in nature. Therefore, all the last three options are incorrect. Because, the atomic mass does not tell anything about the availability or natural abundance of an element.

Now, the isotopes of an element are those elements, which have same number of electrons and protons as the original element, but different number of neutrons. Therefore, they have same atomic number but, different atomic weight or atomic masses.

Hence, by looking at an elements having same atomic number, but different atomic masses, we can identify them as isotopes.

Thus, the correct option is:

A. there is an isotope of lanthanum with an atomic mass of 138.9.