Question

A question about hexagonal closest packing. ? The answer is b

Answer 1

Kind of an odd question, but all the layers consist of a surface one atom thick, so the distance from `"Co"` atom centers within each layer is equal to the distance between centers of nearest neighbors in adjacent layers.

How I interpreted it is that we first measure the distance from center to center of the 7-atom layers...

...and then we consider the distance from center to center of the 3-atom layers. This would be the distance from the center of `A` to the center of `A'` (with `A'` above `A`).