Answer:
The solid part of a rocky planet is the geosphere.
Explanation:
Geosphere; is correct because it is representing the surface and near-surface part of a rocky planet, which is the solid part of it.
Channel; is not correct because it is a term that can be used in several different manners in several different fileds, but it doesn't represent in any sense a solid part of a planet.
System; is not correct because it can refer to any kind of system.
Biospheres; is not correct because it is the sphere representing the living organisms and their habitats.
