Final answer:
Parallel plot lines increase tension when two characters formulate plans against each other, as seen in the scenario where King Richard plots against Richmond while Lord Stanley plots against King Richard.
Step-by-step explanation:
The student's question pertains to how parallel plot lines create tension or suspense in a narrative. From the given options, the event that most effectively demonstrates how parallel plot lines can build tension is: While King Richard plots against Richmond, Lord Stanley plots against King Richard. This scenario creates a suspenseful situation because two characters are devising schemes against one another, unbeknownst to at least one of them. This type of plotting and counter-plotting is a classic device for increasing suspense because the audience is aware of both sides and thus anticipates the inevitable clash.