Answer:
[tex]\boxed {\tt Carbon \ dioxide}[/tex]
Explanation:
Let's think about this from a human perspective (humans are multicellular organisms).
Humans inhale oxygen to use in cellular respiration. Carbon dioxide is the waste product of the process and it is exhaled.
Humans consume proteins and make proteins in protein synthesis. They are used for all sorts of things such as: hormones, repairing tissues, catalysts, providing structure, and more.
So, clearly, the substance that is let out is carbon dioxide. It is exhaled as waste when we breathe, but proteins are used inside the body.
