"at will" means according to one's desire. 
"willfully" means "with the intention of causing harm; deliberately." It is a negative usage.

But isn't going with option A causes parallelism error?

Hi,

'Can't scold at will' is a more relevant phrase in this situation. There is no need for the continuation as you have mentioned.

HI Tapan,
The phrases "cannot scold at will" and "cannot scold willfully" have slightly different meanings:

1. "Cannot scold at will": This means that the person is unable to scold or reprimand someone whenever they want or whenever they feel like it. It suggests a lack of freedom or ability to express criticism or anger whenever desired.
2. "Cannot scold willfully": This means that the person is unable to scold or reprimand someone intentionally or deliberately. It implies a restriction or inability to scold in a purposeful or conscious manner.