Final answer:
During gene expression, the molecule that carries the information that encodes polypeptides is mRNA.
Step-by-step explanation:
The molecule that carries the information that encodes polypeptides during gene expression is mRNA (messenger RNA).
Messenger RNA is transcribed from DNA and carries the genetic message to the ribosomes, where it directs the synthesis of proteins. The base sequence in mRNA specifies the amino acid sequence in the polypeptide chains.
Therefore, the correct answer is b. mRNA.