The student was trying to determine the atomic mass of the element.
Answer: Option B
Explanation:
Isotopes of an element are those which have different number of neutrons but same number of protons. So as the number of protons are same, the isotopes of an element is tend to have similar atomic number but varying mass number or atomic mass.
This is because mass number is determined by the sum of number of protons and neutrons. So in order to determine the atomic mass of an element from its isotope, we can calculate the average weight of all the isotopes. The average atomic mass of all the isotopes will give as precise atomic mass of the particular element.