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The complete graph with $k+1$ vertices (which is strongly $k$-regular) has degree $k$ and chromatic number $k+1$. On the other hand, by Brook's theorem this is the massimum possible value for the chromatic number of a $k$-regular graph, so $$\limsup_k \frac{c_k}{k} = \lim_k \frac{c_k}{k}=1.$$