Oxygen-16 is the right answer as this isotope is not used in any type of radioactive dating, instead the oxygen isotopes are used for determening the processes in the living organisms.
Carbon-14 is not correct because it is one of the most commonly used isotopes for radioactive dating, being the most precise to a certain point of age.
Uranium-235 is not correct because it is an isotope commonly used for rafdioactive dating, especially for much longer periods.
Potassium-40 is not correct because in combination with argon-40 it is one of the main isotopes for determening the age through radioactive dating.
