A

To derive the length of an astronomical telescope for normal vision, we start with the formula for telescope length, which is:
Length (L) = f_o + f_e
Where:
- f_o = focal length of the objective lens
- f_e = focal length of the eye lens.

In a relaxed eye scenario, we set the final image at infinity, and thus, this equation appropriately describes the telescope's effective length for normal vision. This results in Option A being the correct expression as shown in the images.