Forcing children who show a preference for their left hand to use their right hand may not only result in speech difficulties, creating behavioral problems.
A. difficulties, creating behavioral problems
Idiomatically wrong and there is the wrong verb tense, not parallel with the first part of the sentence
B. difficulties but also creating behavioral problems
Similar to A, the -ing form is wrong
C. difficulties but may also create behavioral problems
Correct one? Honestly, I'm not sure, but the others are clearly wrong for a reason or another. I ruled out this in the first time because I thought that another "may" before also breaks the parallelism, but in absence of any option...
D. difficulties; it also creates behavioral problems
The semicolon stops the entire sentence trasforming it in two separate sentences. But the first one does not make sense if it is not linked to the second. Moreover, the idiom is broken
E. difficulties; behavioral problems themselves are also created by it
Similar to D, and the second part is really awkard
I bet on C