Answer:
b. People develop closeness through shared experiences.
Step-by-step explanation:
The theme that is set by the interaction between the two characters is that over closeness reminiscing experiences they had together at their grandmother's house by the water.
Sean does indeed have happy memories of his grandmother, but this is not the principal theme. Sean and Erin are not actually on a dock but flipping through a (presumably) old photo album. There families probably did vacation near the water, but what matters most for the theme here are the bonds that are forged.