The core concept tested here is the understanding of neurotransmitters and their effects on human emotions. The correct answer is C, Gamma-aminobutyric acid (GABA), as it is directly mentioned in the context as responsible for feelings of calm, comfort, and serenity. Dopamine (A) is associated with pleasure and reward, Serotonin (B) with mood stabilization, and Oxytocin (D) with bonding and social interactions, making them incorrect for this specific question.