In the early 1930’s william rose fed laboratory rats a mixture of all 19 amino acids known to be present in proteins. he observed that his rats lost weight. this observation led to the discovery of as the 20th amino acid found in proteins.

If these were the missing choices:
A)Gly
B)Ala
C)Tyr
D)Asn
E)Thr

The answer is E.) Thr.

Thr stands for Threonine. It is a polar amino acid. Its side chains are also hydroxylic.
Aside from promoting normal growth by helping maintain proper protein balance in a body, Threonine also supports cardiovascular, central nervous, immune system, and liver function.

Very hard to understand