Final answer:
Option D best illustrates that Psyche learns the importance of keeping one's word as it shows her weeping and longing for her husband after breaking her promise.
Step-by-step explanation:
To identify the lines that illustrate Psyche learns the importance of keeping one's word in 'Psyche's Promise,' option D, "Psyche wept many tears over breaking her promise. She desperately longed for her husband and searched the earth for him." is the most explicit. It aligns with the theme that it is crucial to keep promises as reflected in both 'The Enchanted Prince' and 'Psyche's Promise.' This option shows the consequences Psyche faces due to not keeping her promise, thereby learning its importance.