Answer:
Apply the sine theorem in triangle ABC:
sin C = AB/AC = 4/5
=> C = arcsin(4/5) = 53.13 deg
As shown in picture, AB//CD. Using the same side interior angles theorem:
=> angle BCD = angle ABC = 90 deg
=> angle ACD = 90 - angle ACB = 90 - C = 90 - 53.13 = 36.87 deg
Apply the cosine theorem in triangle ACD:
cos C = AC/CD
=> CD = AC/cos C = 5/cos(36.87) = 5/0.8 = 6.25
=> Option C is correct
Hope this helps!
:)
