The correct answer is option 4, 16.5 u. The average atomic mass of an element is calculated by taking into account the mass and proportion of each isotope present in a sample. In this case, the sample contains two isotopes of oxygen with masses of 16 u and 18 u, and the proportion of these isotopes is given as 3:1.

To calculate the average atomic mass, we use the formula:

Average atomic mass = (mass of first isotope * proportion of first isotope) + (mass of second isotope * proportion of second isotope)

Using the given proportions, we have:

Average atomic mass = (16 u * 3) + (18 u * 1) = 48 u + 18 u = 66 u

Finally, to find the average atomic mass per oxygen atom, we divide by the total number of isotopes in the sample, which is 4 (3 + 1):

Average atomic mass = 66 u / 4 = 16.5 u

Therefore, the average atomic mass of oxygen in this sample is 16.5 u.