ANS:D - none of these

"None of these" means that none of the options provided in the question are correct. In the context of the question, "none of these" implies that none of the listed pairs of metals occur in nature in their free metallic state. It's important to note that "none of these" is often used as an option in multiple-choice questions when none of the provided options accurately answers the question or if the correct answer is not among the options given. Therefore, selecting "none of these" indicates that the correct answer lies outside of the provided choices.