Answer:
Option B : 16
Explanation:
Refer the attached file 
Given : 
In ΔABC , AB = b , BC = 12 , AC= 20
To Find : value of b i.e. Length of AB
Solution: 
Since we are given a right angles triangle so we can use Pythagoras theorem i.e.
 ---(a)
 ---(a)
Now in a given triangle AB is Perpendicular , BC is Base and AC is Hypotenuse.
So, putting values in (a)







Thus the value of b is 6 
Hence Option B is correct.