See explanation below.
Work Step by Step
The stability of a nuclide is determined from the ratio of number of neutrons For protons, that is N/Z ratio. If N/Z ratio is either too high or too small then the nuclide is unstable N/z ratio of nuclide a is too high, hence it is unstable. N/z ratio of nuclide b is medium, hence it is stable. N/z ratio of nuclide c is too high, therefore it is unstable. This answer can be found in the chapter 24 reading.