Final answer:
The correct answer is the skull. The fontanels on a baby's head are covered with a tough membrane and gradually ossify over time.
Step-by-step explanation:
The correct answer is a) Skull. The soft spots on a baby's head are called fontanels. These fontanels are covered with a tough membrane and allow the head to change shape slightly during birth. Over time, the fontanels gradually ossify and become solid bones of the skull.