A patient was asked to protrude his tongue and salt was placed on the anterior 2/3rd of the tongue. This procedure is used for testing which nerve?

The sensory nerve fibers from the taste buds on the anterior two-thirds of the tongue travel in the chorda tympani branch of the facial nerve, and those from the posterior third of the tongue reach the  brainstem via the glossopharyngeal nerve. The fibers from areas other than the tongue (eg, pharynx)  reach the brain stem via the vagus nerve.