The correct answer is a) ethmoid.
The olfactory foramina are small openings located in the cribriform plate of the ethmoid bone. These foramina allow the passage of olfactory nerve fibers from the nasal cavity to the olfactory bulbs in the brain, which are responsible for the sense of smell. While the sphenoid, temporal, nasal, and zygomatic bones play important roles in the structure of the skull and face, it is specifically the ethmoid bone that is associated with the olfactory foramina.