Final answer:
The cranial nerve associated with the symptoms of mastication weakness, facial anesthesia, and decreased tongue sensation is the trigeminal nerve (cranial nerve V).
Step-by-step explanation:
The symptoms described in the question, specifically weakness or loss of ability to masticate, facial anesthesia (decreased lip sensation), and decreased sensation of the tongue (anterior 2/3), point towards impairment of the trigeminal nerve (cranial nerve V). The trigeminal nerve is responsible for the muscles involved in chewing and provides sensory information from the face to the brain, including the anterior two-thirds of the tongue. Other cranial nerves, such as the facial nerve (cranial nerve VII) and glossopharyngeal nerve (cranial nerve IX), also play roles in facial sensation and taste but the symptoms most closely correlate with the anatomical distribution and functions of the trigeminal nerve.