The correct answer is a) frontal.
The human skull is made up of two main groups of bones: the cranial bones and the facial bones. The frontal bone is part of the cranial bones that form the forehead and the upper part of the eye sockets, whereas the maxilla, vomer, mandible, and zygomatic bones are classified as facial bones.
The maxilla forms the upper jaw, the vomer is a thin bone that forms part of the nasal septum, the mandible is the lower jawbone, and the zygomatic bones are the cheekbones. Therefore, among the listed options, the frontal bone is the only one that is not considered a facial bone.