Which type of figurative language is used in the following line from martin luther king jr.'s letter?

"in some not too distant tomorrow the radiant stars of love and brotherhood."

metaphor
simile
personification - i think
symbol

It's not a personification, those are not living things that are given personalities. 
A metaphor is a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable. So there is no action being applied to the stars, so its not a metaphor.
Simile: a figure of speech involving the comparison of one thing with another thing of a different kind: no
Symbol: a thing that represents or stands for something else: Yes!

Your answer is Symbol, hope this helps!