Of the following sentences, which represents the greatest degree of syntactic complexity?

• A. Is John helping Bill?
• B. Why isn't John helping Bill?
• C. John isn't helping Bill.
• D. Why is John helping Bill?

Answer: (B)

The most complex sentence is the one that combines a fronted wh-word with negation. Syntactic complexity grows when you stack operations like moving a question word to the front and adding negation inside the clause. In these options, two are yes-no questions and one is a plain wh-question, while another is a wh-question that also includes negation.

Why this sentence stands out is that it involves both wh-fronting (the Why moves to the front of the clause) and the negation of the auxiliary (isn't), which means you’re coordinating fronting, inversion, and a contracted negative form all in one structure. That layering of multiple grammatical operations makes its underlying structure more intricate than the others.

The other sentences either lack negation, or are declarative, or are yes-no questions without the added negation layer. So the sentence with both a wh-question and negation has the greatest degree of syntactic complexity.