In the fourth sentence of this paragraph, "they" is a (B.) subject pronoun.
A pronoun is a word that is used to replace a noun that, generally, has already been mentioned. Moreover, there are different types of pronouns since each of them serve a different purpose. Some of them are subject pronouns, object pronouns, possessive pronouns and reflexive pronouns. The pronoun "they" in the fourth sentence of this paragraph is a subject pronoun because it is acting as the subject of the verb "sailed" and it is referring to a specific group of people (the Tuatha Dé Dannan).