The average atomic mass of oxygen is 15.9994 amu. The atomic masses of its three isotopes are as follows: O-16: 15.995 amu; O-17: 16.999 amu; O-18: 17.999 amu Which isotope is most likely to have the greatest abundance in nature?

The average atomic mass of an atom would be the closest to the mass of its most commonly found isotope, because if another isotope would be more common, the average would "shift" to be closer to the most common isotope.

So the answer is O-16: its mass is almost the same as the average atomic mass.