Which structure is associated with tighter coils and is prone to dryness and brittleness?

The correct structure associated with tighter coils and a propensity for dryness and brittleness is the hair structure rather than the bones listed in the options. However, if we consider the context of hair types, typically, tighter coils are characteristic of type 4 hair.

This hair type is naturally drier because the shape of the follicle makes it difficult for the sebum produced by the scalp to travel down the hair shaft. The sharper angles and tighter curls can impede moisture retention, leading to increased brittleness.

In terms of the bones you've listed, while they play a crucial role in the anatomy of the skull, they do not relate to hair structure or its properties. So while the answer may have been chosen based on a misunderstanding of which attributes belong to hair versus skeletal structure, the concept of curled hair types needing special care due to their structural properties can clarify the relevance of the question.