D. his own scientific investigations.
Leonardo da Vinci was known for his keen interest in the human form and anatomy. He conducted his own scientific investigations, dissecting human corpses to gain a better understanding of the structure and functioning of the human body. This knowledge informed his depictions of the human form in his artwork, which were highly detailed and realistic, and helped to establish him as one of the greatest artists of the Renaissance. While he was certainly influenced by the work of other artists and the study of ancient masters, his depictions of the human form were primarily based on his own scientific investigations.