Concept:An apostrophe is a figure of speech where a speaker directly addresses a person who is absent, dead, or an abstract idea.Explanation:The given line is “Milton! thou should'st be living at this hour”.Here, the speaker directly addresses John Milton, who is a dead poet.Since Milton is not present and cannot respond, this direct address is called an apostrophe.It is not an aside, soliloquy, or suspense.Answer:B. apostrophe