The correct answer is: Their motivations can create conflicts that relate to a story’s themes.
Explanation:
Complex characters often have multifaceted motivations that can lead to internal and external conflicts, driving the plot forward. These conflicts are not only pivotal for character development but also directly relate to the overarching themes of the story. When a character's motivations clash with others or with circumstances, it can highlight key themes such as love, betrayal, identity, or morality, thereby enriching the narrative and engaging the reader on a deeper level.
While the other options highlight aspects of character interaction or reader interest, they do not specifically address how a complex character's motivations function within the plot and themes, which is crucial to driving the story.