The question is after a passage. It asks to combine 2 sentences in a proper way.
“That claim may sound incredible. It is based on extensive fossil evidence.”
The best answer is:
“As incredible as that claim may sound, it is based on extensive fossil evidence.”

could you please explain why?

The short explanation is that that choice is grammatically correct and doesn’t change the meaning of the two original sentences. The best way to get a question like this right is to eliminate the other 4 answers (not provided in this ask) because they’ll all either have a grammar error or not fit into the paragraph well in context.