<span>the behavior of the above pair of substances</span> is soluble
For an element whose third shell contains six electrons, the appropriate electron configuration is; 1s2 2s2 2p6 3s2 3p4.
The electron configuration shows the distribution of electrons in the shells of an atom and in orbitals.
We have been told that the six electrons are found in the third shell. This shell has n=3 and the configuration of this shell must ns2 np4.
The only electron configuration that meets this standard is 1s2 2s2 2p6 3s2 3p4.
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KCI is not a covalent compound, it is an ionic compound.
A covalent compound is one in which each of the atoms involved contribute a specific number of electrons for sharing in order to from stable compound while an ionic compound is a compound formed when one atom donates electron to the other atom in the compound, in order to attain stability. The compounds given in options A, B and D shared electrons while in KCl, potassium donates an electron to chlorine.
I'm going on a limb here, but Carbon is a definite. <span />