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

Roll on, thou deep and dark blue Ocean - Roll!

Apostrophe

The correct answer is Apostrophe.

Apostrophe is a figure of speech in which the speaker addresses an absent or inanimate object. In the expression "Roll on, thou deep and dark blue Ocean - Roll!", the speaker is addressing the ocean as if it were a living person. This creates a sense of urgency and power, as the speaker is commanding the ocean to continue its journey.

The other answer choices are incorrect. Metaphor is a comparison between two things without using the words "like" or "as." Oxymoron is a figure of speech in which two contradictory terms are used together. Alliteration is the repetition of consonant sounds at the beginning of words. None of these devices are used in the expression "Roll on, thou deep and dark blue Ocean - Roll!".