Final answer:
The greatest degree of syntactic complexity is represented by option D: Why is John helping Bill?
Step-by-step explanation:
The sentence that represents the greatest degree of syntactic complexity is option D: Why is John helping Bill?
This sentence is more complex because it includes the interrogative word 'why', an auxiliary verb 'is', and a gerund 'helping'. The addition of these elements adds more complexity to the sentence structure compared to the other options.
In contrast, the other options have simpler structures. Option A is a simple yes/no question with just the subject 'John' and the verb 'helping'. Option B is a question with the negative auxiliary verb 'isn't' added. Option C is a simple statement with the negative auxiliary verb 'isn't' added.