As cytokinins are primarily produced in roots, what route would they travel to influence lateral shoot formation in a recently topped tree?

The answer is symplastic transport. Transport of phytohormones such as cytokinins occurs through cytoplasm from cell to cell through plasmodesmata.  Cytokinins travel up the xylem and promote lateral growth. Gaseous molecules, such as ethylene, emitted and received by plants must pass the apoplast.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."