Final answer:
The average atomic mass of lanthanum is calculated by considering the masses and abundances of its isotopes, yielding approximately 139.0 amu, which corresponds to option D.
Step-by-step explanation:
The average atomic mass of lanthanum can be calculated by multiplying the mass of each isotope by its abundance and then summing the results. For the isotope with an atomic mass of 138.9 amu and an abundance of 99.91%, the contribution to the average atomic mass is 138.9 amu × 0.9991.
For the second isotope with an atomic mass of 137.9 amu (corrected from the typo '1379 amu') and an abundance of 0.09%, the contribution is 137.9 amu × 0.0009. Adding these contributions together gives the average atomic mass of lanthanum:
138.9 amu × 0.9991 + 137.9 amu × 0.0009 = 138.88711 amu + 0.12411 amu = 139.01122 amu.
However, since the answer must be rounded to one decimal place as per the provided options, the average atomic mass of lanthanum is approximately 139.0 amu, which corresponds to option D.