When the mass number of oxygen is 16, how many neutrons does oxygen have in its nucleus?

The mass number is the sum of neutrons and protons in a nucleus. 

The atomic number is the number of protons or electrons in an atom. If we know the atomic number and mass number, the number of neutrons can be calculated as;

${\mathrm{N}}_{\mathrm{neutrons}}=\mathrm{Mass} \mathrm{no}-\mathrm{Atomic} \mathrm{no}$

We know oxygen has atomic number 8. And its mass number is given as 16. Thus the number of protons is;

${\mathrm{N}}_{\mathrm{neutrons}}=16-8=8$

The correct answer is 8.