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

Milton! thou should'st be living at this hour.

Apostrophe

The correct answer is OPTION 3 - Apostrophe.

Apostrophe is a figure of speech in which the speaker addresses an absent or inanimate object. In the expression "Milton! thou should'st be living at this hour," the speaker is addressing the poet John Milton, who is long dead. This creates a sense of urgency and longing, as the speaker wishes that Milton could be alive to witness the current events.

The other answer choices are incorrect. Personification is a figure of speech in which inanimate objects are given human qualities. Alliteration is the repetition of consonant sounds at the beginning of words. Oxymoron is a figure of speech in which two contradictory terms are used together. None of these devices are used in the expression "Milton! thou should'st be living at this hour."