In line 20, "elemental" most nearly means D. essential. So, the correct answer is option D.
In this context, "elemental" pertains to the fundamental or essential features discussed in the passage. The term is intricately tied to the concept of cuteness and the baby schema, suggesting that the reactions and responses to cuteness are rooted in these elemental features of stimuli.
These fundamental characteristics, including a large head, a high and protruding forehead, and large eyes, are deemed elemental as they form the core components that elicit the perception of cuteness.
Essentially, "elemental" here emphasizes the foundational and indispensable nature of these features in triggering the emotional responses associated with cuteness, shedding light on the innate and universal aspects of human reactions to certain stimuli that align with the baby schema.
Therefore, option D is correct.