The detail that best contributes to the development of the theme "the most difficult trials come with great reward" is:
4. On one weekday morning after Christmas, six inches of new snow had just fallen. We were standing up to our boot tops in snow on a front yard on trafficked Reynolds Street, waiting for cars.
This detail highlights the challenging setting of heavy snow, which represents a difficult trial for the characters. Standing in deep snow and facing the challenge of waiting for cars can be seen as an obstacle. However, the context of this scene suggests that overcoming this trial—perhaps for the sake of fun or adventure—leads to a greater reward, embodying the theme of how perseverance through tough situations can lead to rewarding experiences.