Explanation:

- The atomic mass of an element, expressed in 'u' (atomic mass units), is the weighted average of the masses of its isotopes, based on their natural abundances.

- Option 1: 24

- This is incorrect for oxygen. The atomic mass of oxygen is not 24 u.

- Option 2: 8

- This refers to oxygen's atomic number, which is the number of protons, not its atomic mass.

- Option 3: 16

- This is the correct atomic mass for oxygen. Oxygen-16 is the most abundant isotope.

- Option 4: 32

- This value is the molar mass of diatomic oxygen gas (O2), not the atomic mass of an individual oxygen atom.