This tests modifier errors. First, to make sense of the sentence, ignore everything from "which have" to "their bodies". The original sentence becomes:

Unlike other arachnids, the scorpion's nerve cells are clustered in its head, like a mammal's.

"Unlike x, y" is a direct comparison in which x and y must be comparable. In this case, answer (A) is incorrect because it compares the scorpion's cells to other arachnids. A better answer would compare the scorpion itself to other arachnids.

We can use this realization to get rid of B (compares "the scorpion's head" to "other arachnids"), D (same as A), and E (compares "a cluster of cells" to "other arachnids".

answer C is correct because it correctly compares the scorpion itself to other arachnids.

Hope this helps. My signature below has more info.
-Patrick