All known proteins in cells adopt a single stable conformation because

Question

(a) any chain of amino acids can fold up into only one stable conformation.
(b) protein chains that can adopt several different conformations have been weeded out by natural selection.
(c) chaperone proteins prevent the protein from adopting a preferred unstable
conformation.
(d) they are complexed with other molecules that keep them in that one particular
conformation.
(e) one conformation always has the most positive free energy.

-I'm going with (b) with this one because that is the reason why the cell uses only 20 amino acids out of the numberous ones out there.