In the structure of the story, the struggle between the loyal friends and the merciless king creates — a. relationships c. motivation b. conflict d. foreshadowing

C. conflict

The main word in what you provided above is "struggle". This would most often imply to a complication, or in this case a conflict. While all the other choices could happen: the people could develop a relationship, this could serve as motivation for another conflict to come, and it could foreshadow the king's or the friends' future action, it is not definite. There is no justification that it will inevitably happen. However, a conflict by definition is a disagreement or a clash which more or less means a struggle. Hope this helps!