Answer:
Option: B is the correct answer.
B. 18 cm and 24 centimeters
Step-by-step explanation:
We know that if two triangles are similar than the ratio of the corresponding sides of the two triangles are equal.
i.e. if there is a triangle with side lengths a,b and c units respectively and other triangle with the corresponding side lengths a',b' and c' respectively.
Then for two triangles to be similar we have:

Here we have:
Juan drew a right triangle with leg length of 6 centimeters and 8 centimeters .
Now we will check in each of the options whose ratio are equal:
A. 9 centimeters and 16 centimeters

and 
As 
Hence, option: A is incorrect.
B. 18 cm and 24 centimeters

and 
As both the ratios are equal.
Hence, option: B is correct.
C. 16 centimeters and 18 centimeters

and 
As 
Hence, option: C is incorrect.
D. 3 centimeters and 5 centimeters

and 
As 
Hence, option: D is incorrect.