The statement that uncombined single elements in their standard state have an oxidation state of zero is a fundamental, foundational rule of chemistry taught in introductory chemistry courses, and therefore requires no citation.
The claim is a definitional rule of chemistry (the standard assignment rule for oxidation states of free elements). According to the prompt instructions, a claim that is trivially true by definition or direct everyday observation is common knowledge, requiring no citation and yielding the COMMON_KNOWLEDGE verdict with empty index arrays.