Id go with C
For me it was between C and E
E is wrong as the sentence almost means - any paleontologist who studies..estimates.....thats not the case here....the sentence says that the ones studying the fossils have estimated! SO E is out.
A is out as only approximately is awkward and verbose!
Don't you think that only and appoximately suggest 2 diff things. It is not redundant. Only significes a small portion whereas appox suggests that the writter is not sure of the exact amount.
These 2 words are not REDUNDANT.