Answer: The effect of the alliteration in the phrase "devil more dm'd" is that it emphasizes the wickedness that Macduff sees in Macbeth. Thus, option B is the correct answer.
Macduff wants to preserve his country, demonstrating his goodness and loyalty to Scotland, which is the antithesis of Macbeth's theme of good versus evil. This reveals Macduff's view that Macbeth is evil and twisted.