Concept:The outer structure of the earth refers to the external spheres or envelopes that surround the solid interior, such as the air, water, and living matter.
Explanation:The atmosphere is the gaseous layer surrounding the earth, so it is an outer structure.
The hydrosphere consists of all water bodies on the earth's surface, so it is an outer structure.
The biosphere is the zone where life exists, combining parts of the air, water, and land, so it is an outer structure.
The mesosphere, however, is not an outer envelope of the earth.
In earth's internal structure, the mesosphere is the lower part of the mantle, lying beneath the lithosphere and above the core.
Therefore, among the given options, the mesosphere is the only part that belongs to the interior rather than the outer structure of the earth.
Answer:Mesosphere.