Which Lewis dot structure is the best representation of the bonding in the thiocyanate ion  ${\mathrm{SCN}}^{-}$ ?

The stability of any resonating structure can be defined in terms of its formal charge. Of all the potential resonating structures, a more stable structure will contribute more to the resonance hybrid. The most stable structure out of all the potential structures is chosen with the use of formal charges.

In structure (D) Formal charges on S, C & N are 1,0, -2

$\mathrm{S}=\mathrm{C}=\stackrel{\ominus }{\mathrm{N}}$