The sentence "John wants to really do well on this exam" contains which of the following errors?

a. dangling modifier

c. split infinitive

b. squinting modifier

d. all of the above

This sentence contains split infinitive. John wants to really do well - you see that the infinitive is split here by introducing really between to and do. If you can, you should avoid doing that, it would be better to say John really wants to do well ...