(A) 10/9
Using the relation: $n_{YX} = \dfrac{n_Y}{n_X} = \dfrac{4/3}{6/5} = \dfrac{4}{3} \times \dfrac{5}{6} = \dfrac{20}{18} = \dfrac{10}{9}$
The refractive index of Y with respect to X equals the absolute refractive index of Y divided by the absolute refractive index of X (both measured with respect to air/vacuum). This follows from the relation $n_{21} = v_1/v_2$, where expressing both speeds relative to air gives $n_{YX} = n_Y / n_X$. Simply divide the two given values correctly.