Core concept: Omitted conjunction in noun clauses. Analysis: 1. The full structure should be 'Studies show (that) regular exercise improves...', making it a that-clause with omitted 'that'. 2. Key identifier: The clause serves as direct object without relative pronouns 3. Error options: A. Adverbial clauses require subordinating conjunctions C. Relative clauses modify nouns D. Conditional clauses use 'if' or 'unless'