C

To minimize chromatic aberration in a combination of two thin lenses with focal lengths \( f_1 \) and \( f_2 \), the distance \( d \) between the lenses should be chosen correctly. The formula to minimize chromatic aberration is given by:
\[ d = \frac{f_1 f_2}{f_1 + f_2} \]
In this case, both lenses being of the same material, we can assume they have equal magnitudes in focal lengths. If we let \( f_1 = f_2 = f \), we can simplify the equation to:
\[ d = \frac{f \cdot f}{f + f} = \frac{f^2}{2f} = \frac{f}{2} \]
Thus, the distance between the two lenses should be equal to half of their focal length to effectively minimize chromatic aberration. Based on the provided options, the correct answer corresponds to this formulation.