Answer:
D. 32.
Explanation:
Remember the formula of atomic mass:
[tex]atomic\text{ mass=atomic number+\# neutrons.}[/tex]If you go to see the periodic table, you can note that the atomic mass of cobalt is 58.9 amu rounded to 59 amu. Let's solve for '# neutrons' and replace the atomic mass (59) and the atomic number (27) (remember that the number of protons is the same that the atomic number), like this:
[tex]\begin{gathered} #\text{ neutrons=atomic mass-atomic number,} \\ #\text{ neutrons=59-27,} \\ #\text{ neutrons=32.} \end{gathered}[/tex]The answer would be that the number of neutrons to make a cobalt atom most stable is D. 32.