There are a number of ways one can discuss archetypes as they relate to the Arthurian legend portrayed in Le Morte d'Arthur. One can examine specific archetypes represented by characters in the story or one can examine the archetype of the hero as expressed in the actions and events affecting a number of different characters. The archetype of the hero is outlined by Joseph Campbell in his book The Hero with a Thousand Faces (which looks at this archetype across world mythologies and cultures). He describes the Hero's Journey as an expression of this archetype, a journey that includes a set of conditions or situations that define a character's life and purpose.
The most obvious example of the hero archetype is King Arthur himself. The "Call to Adventure" occurs when Arthur accompanies his father Sir Ector and brother Sir Kay to a gathering of knights who wish to try and pull the sword Excalibur from a stone. The knight who can pull the sword from the stone will be crowned king. Arthur, who is not a knight and whose true identity (being the son of the brave King Uther) has been hidden from him, inadvertently pulls the sword from the stone. He is overwhelmed with the attention he receives and runs into the forest ("Refusal of the Call"). There he meets the wizard Merlin, who orchestrated the conditions of his conception ("Meeting with the Mentor").
Arthur's journey in this story (one of many detailing the Arthurian legend) mirrors the Heroic Journey archetype every step of the way, making him a classic example of this mythic and literary idea, and providing an iconic model of the notion of the hero in world literature.