I would be cautious in your analysis here. First, I agree that "the overwhelming extent of the flooding" is parallel to "the degree to which rocks were scattered". These are two events that the photographs indicate, both nouns ('extent' and 'degree') and the construction is fine. However, there is a trap in that the phrases don't use exactly the same type of words. Some students tend to overdo parallelism and might reject a perfectly good parallel construction such as that found in answer D.
What I would encourage you to avoid is this:
the degree to which rocks were scattered = degree of scattering
Simply interpreting a phrase as another phrase is risky. The construction was parallel to begin with; we didn't need to repeat 'of'.
Answer B is indeed not parallel, but for different reasons. The first element has a verb ('was'), but the second doesn't. We'd need something like '...indicate that A is B and [that] C is D'. (We might exclude 'that', depending on how clear the meaning was.)
Answer C, on the other hand, is borderline for me. Although D sounds better to me, I'm not sure I can differentiate the nuance of meaning between 'extent of floods' and 'extent of flooding', but I'm pretty sure that GMAT isn't expecting me to either.
