You should reask this question and put a picture so that we can understand it better :)
Isotopes are variants of a particular chemical element which differ in neutron number (neutral particle)and all isotopes of a given element have the same number of protons (positive particle) in each atom.So isotope B of the sodium will have 11 protons, 11 electrons but it may have a wide range in its number of neutrons.
<em />It's radiocarbon dating because in the previous question it says that she is trying to find the amount of carbon-14 and carbon-12 in the fossil