What does a toric transposition show?

The correct choice is the one that specifies "power in principal meridians." In the context of toric lenses, which are specifically designed to correct astigmatism, the term "toric transposition" refers to the method of transforming the prescription parameters from a cylinder and sphere format into a format that describes the power of the lens specifically at the principal meridians.

Principal meridians are the two axes in which the lens provides maximum and minimum power. In toric lenses, the power varies depending on the orientation of the lens, hence why it is crucial to focus on these particular meridians. This allows eye care professionals to better understand the optical properties of the lens and how it will interact with the light entering the eye along those critical axes.

Understanding the power in all meridians or specific locations of the lens surface, such as the center or edge, is not directly relevant in the context of a toric transposition since the focus is on how the cylinder and sphere powers are represented in relation to the principal meridians. The key takeaway is that the principal meridians provide the essential information for the correction of astigmatism as it relates to how the lens is designed and fitted for optimal vision correction.