Single best answer: D

Reasoning: In the neonate the skull bones are thin and lack diploic trabeculae, the styloid process is still a separate cartilage that has not yet fused with the temporal bone, and the large anterior fontanelle remains open. The mastoid process, however, is rudimentary at birth and only enlarges during the first few years of life, so it is definitely not of adult size in a newborn [Weiglein, 1996, PMID 8782317].