it is c. ionic compound just because it says ionic. can't be a crystal since there's no links to an ionic bond connected to crystals (and if it were it'd depend on the elements and the thingy majig itself right) and a covalent bond is an entirely different bond which isn't related to ionic. hope this helped, and i hope you have a good day too! :D
Answer:
3. Equal numbers of protons and neutrons
Explanation:
The nucleus becomes unstable if the ratio of protons to neutrons is less than 1:1 or more than 1:1.5.
The most stable nucleus has a neutron proton ratio of 1:1 which means that they can not release a neutron or a proton to decay.
Nucleus 3 is therefore the most stable.
