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If you go to page 62 in the Eldar Army List section near the back of the book and go to the Wraithguard entry it says in the Squad section that 10 with a Spiritseer can be either Troops or Elite.
Hey, I'm known as Solitaire on the great Forum Heresy Online. Check it out sometime. . .
note that it is only with the spiritseer in the squad that they are given the choice of being troops or elites (they can be either)