The greeks studied charges using gold copper magnetite amber

The given blank can be filled with amber.

Explanation:

The phenomenon of electricity resulted from a basic characteristic of matter, that is, electric charge. The Greeks studied charges with the application of amber. The characteristic presently known as static electricity was familiar to the ancient Greek philosophers. However, the term electricity originated from elektron, that is, the Greek term for amber.  

Amber refers to a resinous mineral that has an application in producing the jewelry. The amber effect came into force by the fact that the small clothing fibers would have got attracted to the amber jewels and would have become tough to withdraw. Thus, from here on the association of amber with electricity came into light.  

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c. electrical charges of ionic bonds.