Question

Which element could likely substitute for Si in a solid solution

Answer 1

I think you are intended to say Carbon. They both belong in the same family.

Answer 2

The answer is carbon.

That is carbon can replaces silicon that is Si in solids.

As carbon and silicon belongs to same group that is group 14. They have similar properties, silicon comes after carbon and have same kind of properties as that of carbon .

So carbon can replaces silicon in the solids.

That is C can replaces Si in solids.