Which term best describes a feeling of deep remorse or regret?

The term that best describes a feeling of deep remorse or regret is "guilt." Guilt is a complex emotional experience that arises when an individual perceives that they have violated their own moral standards or social norms, often leading to feelings of remorse for actions taken or not taken. This emotion typically involves an awareness of having caused harm or distress to oneself or others, resulting in a desire to make amends or seek forgiveness.

In assessing the other options, sadness refers to a general feeling of unhappiness or sorrow, but it does not inherently involve the recognition of moral wrongdoing that characterizes guilt. Pity involves feeling sorrow for someone's suffering or misfortune but lacks the personal accountability associated with guilt. Longing, on the other hand, signifies a strong desire or yearning for something or someone absent or lost, which also does not align with the concept of remorse or regret. Thus, "guilt" is the most accurate term to convey a deep sense of remorse.