Final answer:
The correct sentence is option B: These children usually find a particular aspect of school frightening, such as a school bully or an overcritical teacher, thus, treatment may require a change in school environment.
Step-by-step explanation:
The sentence that is complete and properly punctuated is option B:
These children usually find a particular aspect of school frightening such as a school bully or an overcritical teacher thus, treatment may require a change in school environment.
Option B is the correct choice because it contains a complete subject and predicate and is punctuated correctly. The sentence introduces the idea that children find a particular aspect of school frightening, provides examples of what they find frightening, and explains the potential need for a change in school environment as a result.