PQ is a chord of length 8 cm of a circle of radius 5 cm

Explanation
By given condition we reach to a geometry of circle

Now we join OT and it bisects PQ at R.
Then △TPQ is isosceles and TO is angular bisector of ∠PTO
[ ∴TP=TQ=Tangent from T upon the circle]
 ∴OT丄PQ
 ∴OT bisects PQ
PR=RQ=4
Now we find OR using the pythagorus theorem

Now by the property of right angle triangle we get
∠TPR+∠RPO=90
=∠TPR+∠PTR(∴TPO=90)
∴∠RPO=∠PTR
∴Right triangle TPR is similar to the right triangle PRO by AA
∴
Final Answer
The correct option is option B i.e. 20/3cm