All of the options provided can be true, but if we had to summarize how a complex character drives a story's plot, a. They usually cause conflict and give the story depth and c. Their motivations can create conflicts that relate to a story's theme are particularly significant.
Complex characters often have multifaceted personalities and motivations, which can lead to both internal and external conflicts that propel the plot forward. Their depth adds richness to the narrative, making it more engaging and relatable. However, options b and d also play a role in enhancing the story, as they can contribute to character relationships and reader interest, respectively.
Ultimately, the most impactful ways complex characters drive the plot are through conflict and the exploration of themes related to their motivations.