Which best shows a harmful effect of a mutation?

The correct answer will be option in which mutation is observed in the legs of the frog.

Explanation:

The mutation is a change which takes place in the sequence of DNA. When it takes place in the segment of DNA which can code the trait, it is expressed in the trait.  

These mutation can prove beneficial to the organisms but can also become lethal to the organisms like the one with a mutation in legs of the frog as the legs of the frog helps provide the ability to move as well to swim in the water. if the legs will show a trait of mutation, the frog will not be able to swim and move properly.  

Thus, option in which mutation is observed in the legs of the frog is the correct answer.