are markings used to identify an arrow. A. crests
B. spines
C. index feathers
D. nocks

Index feather

Explanation:

Having only three feathers, their position can't be symmetric to the string. One stays, usually, at 90° from a side, this feather is commonly called "index pen". It usually has a different color from the other two so that it can be found as you see the arrow.