Final answer:
The subject of the question is warmth, which refers to a friendly and welcoming attitude displayed by Dan's parents.
Step-by-step explanation:
The subject of the question is warmth. In the sentence, Dan's parents greeted the speaker with warmth, making them feel like an old friend of the family. Warmth refers to a friendly and welcoming attitude, which Dan's parents displayed towards the speaker. The other options, formality, reserve, and indifference, do not accurately describe the positive and friendly atmosphere created by Dan's parents.