The subplot that would add the most tension to the story is:
one in which the character’s decision has dramatically different effects on a dear friend in each reality.
This subplot introduces emotional stakes and conflicts, as the character must grapple with the consequences of their choices not just for themselves, but also for someone they care about. This could create a sense of urgency and pressure when it comes time for the character to make their final decision, enhancing the overall tension of the narrative.