When are Alleles identical by descent?

Question

A) they are molecularly indistinguishable.

B) they are functionally equivalent.

C) they are derived from the same ancestral copy.

I believe it's the last one, but I am not sure.

Answer 1

Yes, it is the last one - "C) they are derived from the same ancestral copy."

Alleles are identical by descent (IBS) when two or more individuals share similar nucleotide sequences. Similar nucleotide sequences are inherited from common ancestors.