Suppose if all the bones of the skull (facial bones) are fused together, which function would you not be able to carry out?

to masticate food

Explanation:

In adults, all the bones of the skull, except one, are fused together by immovable joints referred to as sutures. The sutures are fibrous joints which hold the bones of the skull together to form a rigid and strong structure that protects the brain. Skull bones fuse around two-years (18 to 24 months after birth). The only skull bone that is not fused by sutures in adults is the mandible, which is used to masticate food.