Explanation:

The pair 'नैसर्गिक' (Naisargik - natural) and 'कृत्रिम' (Kritrim - artificial) are direct and clear antonyms. While 'जड़-चेतना' (inert-conscious) and 'निर्दय-सदाशय' (merciless-kind-hearted) are also valid antonym pairs, 'नैसर्गिक-कृत्रिम' is a very common and distinct pair.