An atom of strontium has at least four different isotopes. what is the difference between an isotope of 88 sr and an isotope of 84 sr four electrons four protons four neutrons two protons and neutrons

Option C= Four neutrons

Any of two or more species of atoms of a chemical element with the same atomic number and nearly identical chemical behavior but with differing atomic mass or mass number and different physical properties.

Isotopes have the same number of protons and electrons but different numbers of neutrons and therefore have different physical properties.  

That’s why 88 Sr has four more neutrons as compared to 84 Sr.

The isotope of strontium has at least four different isotopes. The difference between an isotope of and an isotope of will be of four neutrons. As isotope has same atomic mass, but different mass number. An isotope has same number of protons but different number of neutrons. In case, of strontium 88 and 84; there are four neutrons less than .

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)