If $\dfrac{2}{3} = \dfrac{4}{6} = \dfrac{6}{9}$, then $\dfrac{2+4+6}{3+6+9}$ equals—

Each fraction reduces to $\dfrac{2}{3}$, so by the sum rule $\dfrac{2+4+6}{3+6+9}=\dfrac{12}{18}$. Since $\dfrac{12}{18}$ also reduces to $\dfrac{2}{3}$, both options (a) and (c) are correct.