Final answer:
Line 5 of the poem 'Design' contains a caesura, which is a pause or break within a line of poetry, exemplified by the em dash.
Step-by-step explanation:
A caesura is a pause in a line of poetry, often found in the middle of a line, which can give the reader a moment to reflect or to add dramatic effect to the poem. In the poem 'Design' by Robert Frost, the line that contains a caesura is option b) line 5: 'Mixed ready to begin the morning right, / Like the ingredients of a witches' broth—'. Here, the caesura is represented by the punctuation mark—the em dash—which induces a pause within the line, contrasting the grim imagery of 'death and blight' with the quotidian activity of beginning the morning. This line also incorporates a play on words with 'right' suggesting the ritual 'rite' of a witches' concoction, adding to the eerie and unsettling atmosphere of the poem.