Let's go through each of the options that were provided to the question stating what part of the circle was highlighted in red and talk about what they mean.
First off, radius is the distance from the outer line to the middle which perfectly incapsulates what the red line is. Therefore, the correct answer would be option A, Radius.
However, if you want to understand the rest of the definitions you can follow with the bullets provided.
Chord - This isn't a math term
Diameter - This term is actually quite similar to that of radius but instead this is the length from one side of the outer line to the other side of the outer line. This would cause the diameter to be twice the length of the radius and so if we want to determine the radius using the diameter we can just divide it by 2
Center - This just description the dot in the middle of the circle
