Answer
c) 121.76 amu is the correct answer.
Work Step by Step
As we know ,
average atomic mass = $\frac{(mass \space of \space isotope A \space \times \space percentage \space of \space natural \space abundance) + (mass \space of \space isotope B\space \times \space percentage \space of \space natural \space abundance) }{100}$
Given,
mass of isotope A = 120.904 , natural abundance = 57.21%
mass of isotope B = 122.904 , natural abundance = 42.79%
now we calculate the average atomic mass of Sb,
average atomic mass =$\frac{(120.904\times57.21) + (122.904\times42.79)}{100}$
average atomic mass =$\frac{7031.33784+5259.06216}{100}$
average atomic mass = $\frac{12175.98}{100}$=121.7598 amu
average atomic mass $\approx$ 121.76 amu
