The similarity or difference between two circles centers on their radii and diameters. Segment AD's length is unknown without further information, and a diameter is any segment passing through the circle's center and touching the circumference in two places. The diameter is double the radius of the circle.
To address the question about the similarity and differences between two circles with centers A and F, we must look at several properties of the circles. Without specific details, we can say that if the two circles are identical, then the radii, diameters, and circumferences would be the same. However, if they are different, then those measurements would vary.
The length of the segment AD cannot be determined without additional information. Typically, if AD is a radius or a diameter, we would need the measurement of either to calculate its length. If AD is a diameter, its length would be twice the radius.
Regarding the diameter of the first circle, any segment that passes through the center of the circle (assuming A is the center) and touches two points on the circumference would be a diameter. The length of the diameter can be determined if we know the radius (it is twice the length of the radius).
Following one of the clues provided, to fit a circle inside of a square so that the side length of the square is equal to the diameter of the circle (a = 2r), we can infer that the perimeter of the circle would indeed be less than the perimeter of the square.