From the given options, select the Figure of Speech (Poetic Device) used in the expression:

His honour rooted in dishonour stood. And faith unfaithful kept him falsely true.

Oxymoron

The correct answer is Oxymoron.

An oxymoron is a figure of speech in which two contradictory terms are used together. In the expression "His honour rooted in dishonour stood," the words "honour" and "dishonour" are contradictory. This creates a sense of irony, which is appropriate because Lancelot's honor is compromised by his love for Elaine.

The other answer choices are incorrect. Metaphor is a comparison between two things without using the words "like" or "as." Simile is a comparison between two things using the words "like" or "as." Apostrophe is a figure of speech in which the speaker addresses an absent or inanimate object. None of these devices are used in the expression "His honour rooted in dishonour stood."