What term describes a deep feeling of sadness or longing?

The term that best describes a deep feeling of sadness or longing is "pine." This word is often used to express a profound yearning for someone or something that is missing, which can encompass feelings of deep sadness or nostalgia. When someone "pines" for someone else, they are experiencing a mix of sorrow and longing, indicating an emotional connection that is unfulfilled.

The other choices do not convey the same depth of emotional resonance. "Plod" refers to moving or walking heavily, suggesting a sense of slow movement rather than any emotional state. "Plea" denotes a request or appeal, usually for help or guidance, rather than a feeling of sadness. "Pivotal" describes something of critical importance or a crucial turning point, which does not align with the emotional aspect described in the question. Hence, "pine" is the choice that accurately captures the essence of a deep feeling of sadness or longing.