Feelings of ardor can best be described as:

Feelings of ardor are characterized by warmth and intensity, often associated with passionate emotions or a strong enthusiasm towards someone or something. The term "ardor" implies a depth of feeling that is vibrant and fervent, suggesting a glowing enthusiasm or passionate love. This aligns perfectly with the idea of warmth and intensity, which encapsulates the emotional richness associated with ardent feelings.

In contrast, the other options denote feelings that lack the fervent quality inherent in ardor. For instance, being cold and unfeeling expresses a sense of distance or lack of emotion, which starkly contrasts with the heated nature of ardor. Indifference also suggests a complete lack of passion or strong emotions, further deviating from the concept of ardor. Similarly, feelings that are quickly fading would not be intense or enduring, which is crucial to the essence of ardor. Thus, warmth and intensity are the most fitting descriptors for feelings of ardor.