Final answer:
Option a is a noun phrase, option b is a verb phrase, option c is an adjective phrase, and option d is a noun phrase.
Step-by-step explanation:
In option a, "Camelot offered an ideal setting in which to govern" is a noun phrase because it functions as the subject of the sentence.
In option b, "Many of the most common ideas about Arthur can be traced to the early Middle Ages" is a verb phrase because it functions as the predicate of the sentence.
In option c, "The deeds of other heroes… soon became associated with Arthur’s name" is an adjective phrase because it modifies the noun "deeds".
In option d, "the writer Nennius mentions Arthur as Dux Bellorum, or leader of battles" is a noun phrase because it functions as the subject of the sentence.