Lassitude refers to a state of physical or mental weariness or lack of energy. Lethargy, which means a lack of energy or enthusiasm, is the most appropriate synonym for lassitude. Piquant means having a strong, spicy, or tangy taste, and is unrelated to lassitude. Tenacity refers to the quality of being determined or persistent, which is the opposite of lassitude. Athleticism refers to the physical abilities and skills related to sports or athletics, which is also unrelated to lassitude. Therefore, lethargy is the correct answer as it best captures the meaning of lassitude.