Ionic bonds form between and
a. ions, atoms
b. atoms, elements
c. metals, nonmetals
d. molecules, compounds

I believe it is metals, nonmetals. that is what I remember from my classes. But I'm going to look it up to be sure.

"Ionic bonding is the complete transfer of valence electron(s) between atoms. It is a type of chemical bond that generates two oppositely charged ions. In ionic bonds, the metal loses electrons to become a positively charged cation, whereas the nonmetal accepts those electrons to become a negatively charged anion. Ionic bonds require an electron donor, often a metal, and an electron acceptor, a nonmetal."

https://chem.libretexts.org/Core/Organic_Chemistry/Fundamentals/Ionic_and_Covalent_Bonds

that verified it is a metal and nonmetal.

The answer is joseph is observing the colour of the reaction mixture to see whether proteins are present in the given solution.