Which word describes a humble plea?

The term "supplication" is appropriately described as a humble plea, as it specifically refers to the act of earnestly and humbly requesting something, often in a religious or pleading context. It implies a deep sense of vulnerability and a sincere appeal for assistance or mercy, often directed toward a higher power or authority.

In contrast, the other terms do not convey this same sense of humble request. "Compunction" refers to a feeling of guilt or moral scruple that prevents or follows the doing of something bad, which is unrelated to making a plea. "Expedient" describes something that is convenient and practical but possibly improper or immoral, indicating a focus on outcomes rather than on a humble request. "Lustration" pertains to a form of purification or ritual cleansing, which again does not connect to the notion of making a humble plea. Thus, "supplication" stands out as the correct answer due to its specific meaning aligned with humility and earnestness in requesting something.