A forest plantation is laid out in blocks or compartments, and every management activity carried out inside it, from thinning to harvesting, depends on being able to move people, tools, and produce across the site without damaging the soil. When land is cleared and planted with rows of trees, the exposed strips between compartments and the disturbed ground around them are highly vulnerable to being cut up by uncontrolled surface run-off.
A properly built access road is constructed with a camber and side drains that intercept rainwater flowing off the plantation and channel it safely away in a controlled path, rather than letting it collect and gouge gullies through the planted rows. The road network also breaks the plantation into smaller blocks, which shortens the distance water can build up speed and volume before it is diverted, further limiting soil loss. This is why control of erosion is considered the best justification for access roads in a forest plantation: the road itself is an engineered drainage feature that protects the soil, not merely a convenience route.
Fire fighting, weed removal, and fertilizer application all benefit from having roads available, but none of these activities depends on the road's physical design in the way erosion control does. A plantation could still be weeded, dressed with fertilizer, or defended from fire using paths that are far less carefully engineered; it is the drainage and land-protection function of a well-built road that makes it indispensable to the survival of the plantation soil over the life of the crop.
When a question asks for the "best" reason among several plausible uses of the same structure, look for the answer that describes a function built into the structure's design itself, rather than one that the structure simply makes easier.