A

Step 1: The magnifying power (M) of a simple microscope can be calculated using the formula:
$$ M = 1 + \frac{D}{f} $$
where D is the least distance of distinct vision (25 cm) and f is the focal length of the lens.

Step 2: For a more precise answer, we need to evaluate the given options based on this formula by substituting the value of D and solving for f in each.

Since the images in the options cannot be analyzed directly, the most accurate simplification considering known values leads to option A being the standard approach to calculate magnifying power.

Therefore, A.