It is necessary to calculate the distance in between the points C and D, in order to determine which is the closest length. Use the following formula:
d = √((y2 - y1)²+(x2 - x1)²)
where (x1,y1) and (x2,y2) are the coordinates of the points.
In this case, you have:
(x1,y1) = C(1,2)
(x2,y2) = D(6,8)
replace the previous values of the parameters into the formula for d and simplify:
d = √((8 - 2)²+(6 - 1)²)
d = √((6)²+(5)²)
d = √(36 + 25)
d = √(61)
d ≈ 7.8