Which word indicates behavior that is primarily motivated by lust?

The word that best indicates behavior primarily motivated by lust is "lascivious." This term specifically refers to actions or attitudes that are overtly sexual or driven by an excessive desire for sexual pleasure. It conveys a sense of lewdness or an inclination toward lustful behavior that is particularly noticeable and, often, socially inappropriate.

In contrast, the other words in the choices do not embody this specific connotation. For instance, "languorous" describes a state of weakness or fatigue, often characterized by dreamy or lazy qualities. "Lugubrious" refers to looking or sounding mournful, often to an exaggerated degree, and is associated with sadness rather than desire. "Amity," on the other hand, signifies friendship or goodwill between people, having no relation to lust or sexual motivations. Thus, "lascivious" clearly stands out as the word that captures the essence of lustful behavior.