If two parents display distinct forms of a trait and all of their offspring (of which there are hundreds) display the same new form of a trait, you would be justified in concluding that:

There has been a mutation and that mutation is a dominant trait. As both the parents have displayed the trait, there has been some reaction to the local environment that has improved or increased the survivabilty of the offspring related to this trait, so much so that the trait is now a characteristic that is dominant in the gene pool.

Explanation:

index fossils are important for index fossils because

they use them to match up rock layers in certain locations

that may be far apart!

So I think it's C