#### Answer

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.