The earth as a whole is a system and each sphere is part of this system, so in the summary, the atmosphere is marked as a boundary, now the atmosphere is considered as a layer of gas that envelopes a planet and its part of it as is retained by the gravity of the planetary body itself, so even when it seems logical to make a distinction between those strata in the planetary body (lithosphere, hydrosphere, and biosphere) and the gas layer (atmosphere) this las actually is part of the planet. So taking into consideration this information the earth as a whole is the component that we are classifying and each sphere sets a boundary, leading us to option B.