The correct answer is option (D):
Tranquility
The relationship between "Emotional" and "Arousal" is one of cause and effect or association. An emotional state can lead to or be characterized by arousal, meaning excitement or activation. We are looking for a word that relates to "Calm" in the same way.
Let's examine the options:
* Humility: Humility is a state of being modest or respectful. While a calm person might display humility, the relationship isn't direct in the same way as the emotional/arousal pair.
* Mildness: Mildness refers to gentleness or lack of intensity. While it can be associated with calm, it is not the best fit.
* Kindness: Kindness is a quality of being friendly, generous, and considerate. It is not necessarily linked to calmness.
* Tranquility: Tranquility signifies a state of calmness and peace. This is the best fit. Calmness *is* Tranquility, the same way Emotion is related to Arousal
* Gentleness: Gentleness implies being kind or tender. It is not equivalent to calmness.
Therefore, "Tranquility" is the best answer because it describes a state of calmness, mirroring the relationship between emotional states and arousal.