As with many complicated SC problems, the first thing you have to figure out is WHAT THE SENTENCE IS TRYING TO SAY. In many cases this is straightforward, but not here. The process:
(1) "2000 years as old as" is nonsense. If you don't see why, try applying it to a statement about other ages: "I am about 4 years as old as my brother." Huh?
This is good enough to eliminate all of A, C, E.
The only reasonable interpretation is that the monuments in Brittany are 2000 years OLDER than the other ones. There's no other way in which this sentence could legitimately make sense.
(2) To break the deadlock between B and D, we need to use the MEANING of the sentence. Here's the meaning of each choice.
Choice B ('supposed Mediterranean predecessors') --> There's no doubt that the other monuments are Mediterranean, but what was supposed (and now disproved by the new evidence) is that they were the predecessors of the ones in Brittany.
Choice D ('supposedly Mediterranean predecessors') --> There's no doubt that the other monuments are the predecessors of the ones in Brittany, but what was supposed (and now disproved by the new evidence) is that they were Mediterranean.
B makes sense; D doesn't. If you don't immediately see why, consider that the decisive evidence was carbon-14 dating (which is used to tell how old things are, not where they're from).