An achromatic convergent doublet of two lenses in contact has a power +2 D . The convex lens has power +5 D . The ratio of the dispersive powers of the convergent and divergent lenses is (in magnitude)

The total power of the achromatic doublet is $P = P_1 + P_2$, where $P_1 = +5\,\text{D}$ and $P = +2\,\text{D}$. Therefore, the power of the second lens is $P_2 = 2 - 5 = -3\,\text{D}$, indicating it is a divergent lens.

For an achromatic combination, the dispersion condition is $\omega_1 P_1 + \omega_2 P_2 = 0$. Substituting $P_1 = +5$ and $P_2 = -3$, we have $5\omega_1 - 3\omega_2 = 0$, which simplifies to $5\omega_1 = 3\omega_2$.

The ratio of the dispersive powers is $\frac{\omega_1}{\omega_2} = \frac{3}{5}$. This corresponds to a ratio of $3:5$.

Final answer: $\boxed{\text{A}}$