In "Macbeth," what literary device is used when Lady Macduff tells her son that his father is dead? Alliteration, assonance, an aside, or figurative language?
None of the above, really: it's an ironic metaphor, and the language it is stated in is colloquial, staccato, plain, and almost entirely monosyllabic.
Sirrah, your father's dead.(35)
And what will you do now? How will you live?
What Lady Macduff is doing is ironically suggesting that her husband's absence makes him, as we might say, "as good as dead" to her and her family.
She also uses it to hypothetically ask her son - again, more irony - how he will live now that his father is no longer here to support them. This emphasis on Macduff's absence focusses the danger Lady Macduff and her children are in, shortly before they are slaughtered by the murderers.