Final answer:
The cranial nerve assessed by a nurse when an infant sucks on a pacifier is the trigeminal nerve (Cranial Nerve V), which is responsible for the muscles and sensations required for sucking.
Step-by-step explanation:
The nurse is assessing the ability of an infant to suck on a pacifier, which involves testing the functionality of cranial nerves. The correct cranial nerve that is responsible for the sucking reflex is the trigeminal nerve (Cranial Nerve V). This nerve is a mixed nerve that carries the general somatic senses from the head, and it controls the muscles required for sucking. While the facial nerve (Cranial Nerve VII) is responsible for taste as well as salivation in the anterior oral cavity and has preganglionic parasympathetic fibers, and the glossopharyngeal nerve (Cranial Nerve IX) participates in taste sensation and the gag reflex, it is the trigeminal nerve that enables the movement and sensations required for sucking on a pacifier. Therefore, the correct answer to the nurse's assessment of an infant's ability to suck on a pacifier is B) Trigeminal.