Which one of the following sentences is an example of Apostrophe?

The correct answer is Option (4) → Milton! Thou should'st be living at this hour.

Apostrophe (figure of speech):

• Apostrophe is a figure of speech in which the speaker directly addresses an absent or dead person, an abstract idea or a thing, as if it could hear.
• It is often marked by a name or "O" followed by an exclamation, and by the old pronoun "thou".
• This sentence: the line opens William Wordsworth's sonnet "London, 1802". The poet speaks directly to John Milton, who had died more than a century earlier, as if he were present.
  Example: O Death, where is thy sting?

Why the Other Options are Incorrect?

Option 1: Man proposes, God disposes. — Incorrect. This proverb is an antithesis: two contrasting ideas set side by side in a balanced form. No one is addressed.
Example: Speech is silver, silence is golden.

Option 2: Such an innocent arch, so cunningly simple. — Incorrect. "Cunningly simple" joins two words of opposite sense, which makes it an oxymoron. The line describes something; it does not address anyone.
Example: The hall fell into a deafening silence.

Option 3: He has fallen asleep. — Incorrect. Said of a person who has died, this is a euphemism: a mild expression used in place of a harsh or unpleasant one ("died").
Example: His grandfather passed away last winter.

Contrast: apostrophe speaks directly to someone absent ("Milton!"); antithesis, oxymoron and euphemism describe or soften an idea without addressing anyone.