The correct answer is the third option: They serve as communication portals between neighboring plant cells, allowing the passage of small molecules between the two cells.
Plasmodesmata are tiny channels that link the cytoplasm (the jelly-like "stuff" in the cell, minus the nucleus) of adjacent plant cells. By doing so, they create bridges between these cells. Plasmodesmata are so important, because they allow for certain functions like facilitating the transportation of water between cells.