There’s a lot more to Willie than one would think: still waters run__________
Correct answer: A
Knowledge of idioms
Still waters run deep (idm): It is the discreet who are the deep; impregnated with the underground that punched the elephant to death.
The remaining answers: Not correct idiom structure.
There’s a lot more to Willie than one might think: impregnated with underworld and punching elephants to death.