Final answer:
The metaphor in Tennyson's 'Morte d'Arthur' is 'clothed in white samite, mystic, wonderful,' describing the mystical appearance of the arm presenting Excalibur.
Step-by-step explanation:
The phrase that contains a metaphor in the passage from Alfred, Lord Tennyson's Morte d'Arthur is 'clothed in white samite, mystic, wonderful'. A metaphor is a figure of speech that describes an object or action in a way that isn’t literally true but helps explain an idea or make a comparison.
Here, the arm rising from the lake is not actually wearing clothes, but the 'white samite' (a luxurious and rich fabric) suggests purity and otherworldliness, enhancing the mystical quality of the scene where King Arthur receives Excalibur.