Answer:
A
Step-by-step explanation:
Determine the mass number for each of the isotopes, which is obtained by adding the number of protons and the number of neutrons. Hence, isotope A has a mass number of 1, isotope B has a mass number of 2, and isotope C has a mass number of 3. To determine the average atomic mass, we multiply the atomic mass of each isotope by its abundance. Hence, we obtain (0.99985 × 1 amu) + (0.00015 × 2 amu) + (0.00 × 3 amu) = 1.00 amu