Answer: Option D :3.85 units.
Step-by-step explanation: Diameter is the longest chord of any circle . It is a straight line passing through the center of the circle and touches the circle at two points. Radius is a line drawn from the center of circle to the circumference. Radius is half of diameter.In the given figure the blue line is diameter .The diameter is the blue line which equals 7.7 units.Radius is half of diameter .Radius = 7.7÷2=3.85 units.
Option D is the right answer.