I believe that the best answer to this question is d. amino acids.
Solid because it maintains their shape and is rigid
in this case the answer would be letter "D"
Default network
When the brain is unoccupied, an fMRI indicates that blood continues to flow via a web of brain regions called the default network.
The default network is a web of brain regions that have activity that corresponds greatly with each other and different from other networks within the brain. The default network is active when an individual's attention is not concentrated on the external environment and it is measured with the functional magnetic resonance imaging technique (fMRI).
External skeleton is the correct answer. Hope this helps out !
