Final answer:
The painting that demonstrates the rococo characteristic of secular themes with a lighthearted element is Pilgrimage to Cythera by Antoine Watteau.
Step-by-step explanation:
The painting that demonstrates the rococo characteristic of secular themes with a lighthearted element is Pilgrimage to Cythera by Antoine Watteau.
Rococo painting during this period typically featured playful themes, ornate colors, and a focus on love and life outside of mythology. Watteau's painting exemplifies these characteristics through its soft application of paint and depiction of classical themes centered around youth and love.
Therefore, Pilgrimage to Cythera is the best painting that illustrates both the rococo characteristic of secular themes and a lighthearted element.