ANS:C - widely differing electronegativity.

The two elements required to form a substitutional solid solution should ideally have similar atomic sizes. This is because substitutional solid solutions involve the replacement of atoms of one element by atoms of another within the crystal lattice. If the atomic sizes of the two elements are significantly different, it can lead to lattice distortion and strain, which may result in a less stable or even non-existent solid solution. Therefore, among the options provided:

• Having the same crystalline structure is not a requirement for forming a substitutional solid solution. Different crystalline structures can still accommodate solid solution formation if the atomic sizes and other factors are suitable.
• Having the same valency is not a strict requirement either, as long as the atoms can substitute for each other in the crystal lattice without causing significant lattice strain.
• Having widely differing electronegativity may affect the bonding behavior between the atoms but does not necessarily prevent the formation of a solid solution, especially if other factors like atomic size compatibility are favorable.