Which sentence uses dashes correctly? a. i need -- a lot of -- colorsfor this painting. b. i can't believe my own --brother! -- is now mayor. c. tom and i -- are planing -- a wedding in june. d. i looked under the bed -- can you believe it? -- and there it was!

D is your answer. Dashes are used to separate parts of the sentence that don't quite belong there, or are useless information that you could get away with not including in the sentence. A good way to think about it is that generally anything you think could be surrounded by parenthesis could also be put in dashes.